You guys got the videos in the other solutions, so I thought about sharing the experience and some tips.
First off, let’s get real. If you are a modest/below average gamer in terms of skill or if you just don't have the patience to keep retrying difficult things, you might want to find yourself a good co-op partner, let him/her host the session (it seems that whoever is hosting experience less lagging in this game) and do the hard tricks for you. But if you are an average gamer like me, who can pull your own weight on Legendary and perform a difficult jump every now and then, you might be just okay for this.
I did 7 out of the 10 levels by myself and some of them in good time. In fact I did all of them solo first, then noticed that it would be better to use some help in a few of them to cut time shorter. My last co-op partner (I played with different people as friends were available to lend a hand) only needed to help me cut a total of 9:35 minutes of any of those levels that took me longer to complete solo in order for this achievement to unlock. It's not an easy cheesy task, but it's definitely possible. You can thank mr. Goat Rope and his charitable work for the existence of this achievement after you've unlock it.
Second, you don't have to execute ALL of the tricks you see the "pros" performing in their videos. Some, however, are inevitable. After many tries I gave up, for example, on the triple grenade jump in The Truth And The Reconciliation (a.k.a. mission 3 or T&R). On the other hand, I practiced and was able to pull off the "shield bump" in the level Keyes. So, that's why I've said, you don't have to be 100% like the pros, but you gotta be able to perform some of them tricks, specially if you're doing this solo.
Third, let's get real about the weapons. The Assault Rifle sucks. There it is, I've said it. I don't understand why that would be the weapon of choice of the Master Chief. It's only good to kill the flood spores and noobs on multiplayer. Drop that "weapon" as soon as you can lay your hands in any of these: Plasma Pistol, Magnum, Shotgun, Rocket Launcher. Those are the 4 weapons that matter (and eventually the sniper, if you start with it or if there's something specific in the level for you to snipe). I mentioned the Plasma Pistol first because I think it is the most overlooked and underrated of them all. But those meanie green shots can sometimes take a grunt with one charge, take down the annoying Jackals' hand shields as well as the Elites' armor shields. Combine it with the magnum and you all know it: noob combo. By the way, the magnum is great, it can even take Banshees down, but even the magnum, sometimes, might not be the best weapon to have on you, if you must choose among other options. So you gotta know what you need, when you need it and sweep the floors for the best weapons. Just don't keep the Assault Rifle if you don't have to. Please.
Now let's move on to specific level's advice:MISSION 1 - PILLAR OF AUTUMN
There is not a lot to comment here. This is one of the easiest missions because it can be done solo in 10 minutes or less, and the fact that you're in a closed area helps a lot to stick enemies with grenades or just blow them up with the regular frag ones.
I've followed mostly the Nak3d Eli videos (and I'm gonna refer to them) that are posted here in one of the other solutions as well as Youtube. Sometimes I also would look up on Youtube for different approaches and there I learned that these tricks have been around far longer than these new videos.
Anyways, of all the tricks that Eli mentions, the staircase jump he does (minute 3:51 in his video) doesn't need to be a crouch jump. A regular jump will do if you perform it right. Besides, if the 2 Elites are blocking your way to the next session, instead of going to the right after the jump, it's okay to take a left and go all the way around. You might deal with a grunt, but that's far easier. However, rushing past and ignoring all of those guys is best.
The most important "trick" on this level IMO, besides the confidence that comes with practice, is the plasma grenade that you must throw on the ground to allow you to pass through the Elites and grunts (minute 5:21 in his video). It's likely that you'll die in the first attempts, but it's very possible to pull it off. As he says in the video, make sure you have the plasma pistol ready for the Elite in the corridor, then switch to the magnum and finish him off. If the enemies behind you already recovered from the plasma grenade you threw and are shooting at you, you can take cover on the right by the glass where you can see the initial room where the Chief started. Wait until your shield recovers and keep going (Eli didn't do that in his video but you might need to, if you're about to die). Waiting for the shields to recover is almost always best than dying and having to redo the whole part.
There's nothing else special enough to be commented about this level. My solo time wasn't the best among my friends, 9:22 minutes, but when I started redoing the levels to save any extra time(every minute counts!) I reduced it to 8:26, which is my personal best for now (and probably forever since I don't plan to keep redoing it).MISSION 2 -HALO
This is also one of the less difficult missions. My time was 24:31 (solo) and then 19:51 (co-op).
I did NOT do the recommended grenade jump on classic mode at the very beginning of the level, so the 2 Banshees as well as the Elite and 4 grunts spawned on my way after I crossed the bridge. No big deal, as one of the Banshees come towards you when the Covenant dropship approaches and if you take that banshee down with the magnum the other one flies away. Then I took the grunts from a distance and dealt with the Elite alone. If the Elite is too aggressive you can get past him by stealthly moving forward using the rocks and trees on the right as cover. Alternatively, if you can get your hands on one of the dead grunts plasma pistols, you can dispatch the Elite with a noob combo.
After that I followed Eli's video for the next part where the 5 dropships land. If you find it too hard to stick the elites before they jump out of the ship, throw plasma grenades on the ground, just before they land. That should get you the same results. Also, when for some reason I failed to get rid of the enemy waves in time and was late for the final 2 dropships, I've found it easier to just go to the top of the main structure and fight them from there. Often they will line up in the ramp when coming after you, which is great for sticking them or using the Needler.
For the next part, I drove the marine all the way to the far side of the wall where the button for the bridge is. Then jumped on his head and above to reach the button, activated the bridge and came back. It doesn't matter if he dies or not in the process. If you're doing this mission co-op, one of you can drive off as soon as the other pressed the button. The person left behind will spawn back close to the warthog once you reach the existing road ramp.
Then I followed the rest of the guide more or less to the letter. Warthog sliding is a bitch and I was able to pull that off without practicing too much just a couple of times. But that's not necessary as you can just run past the jackals and the red Elite to get inside the structure and grenade kill the first set of marines.
When I replayed this mission co-op we split up and one of us went with the warthog to run over the marines on the right side above a rock (get the sniper rifle after killing the guy) and the other went to the left on foot to grenade kill those first marines. Then we met at the third group, which you don't have to betray/kill and started picking the Elites, jackals and grunts from a distance with our sniper rifles before moving in.MISSION 3 - THE TRUTH AND THE RECONCILIATION
Now, this is where my pain and fears started. As I watched Eli's video and saw he pulling off the infamous triple grenade jump (actually I've found a guy on Youtube who did a single amazing long jump), I doubted that I could replicate it on Legendary. Maybe I could, but that would take A LOT of practice and I don't intend to keep playing just this game for that long. The other challenging parts on this mission playing solo were the countless waves of enemies coming down the elevator exterminating me and the marines, the following sequence with the invisible sword Elites and finally making sure that Keyes didn't rush and die later on in the room with the 3 invisible Elites. After playing the level over and over I got better at all that.
Start this mission by following the video guide, sniping at least the 2 jackals and the 2 Elites from afar. If you decide to progress without killing all the other grunts (I did that to save ammo and time) a 3rd Elite will come from around the far corner. Kill him too.
In the open area just before reaching the sleeping grunts and the elevator, I just sniped the 2 red elites and that was all I've killed before picking the invisible camouflage and ran to assassinate the little guys in the next area.
There, and with practice, I was able to kill the enemies at the elevator part as they came down. Having grenades and throwing them properly as well as sniping the Elite of each wave first indeed help a lot. If you loose control of the situation while handling a wave, take a good cover and start picking the enemies one by one. Plasma pistols are also great for all sorts of enemies, except the Hunters. As soon as those show up, run towards them (that's right) so they won't use their green explosive rocket stuff on you. It's easy to make them charge at you and then dodge to the side before hitting them once in the exposed back with the sniper.
The next part after the elevator is also hard to pull off solo but with practice and lots of attention and grenades you can kill all the grunts and jackals while dodging and fighting the invisible sword Elites. Sniper rifle and plasma pistol are the way to go there.
Then you pick the active camo and run past lots of enemies and pray that one of the doors will be open. If for some reason you're unlucky, reload the checkpoint or try pushing one of the enemies closer to the door to get it open, which isn't the best way since if you start a fight all the others will shoot you, but I've managed to pull that trick off a couple of times.
The following part is where you supposedly do the triple grenade jumps. If you, like me, can't do that, then you'll have to quickly kill all of the enemies here so the Hunters finally show up so you can dispatch them and move on.
After you rescue Keyes, Eli suggests in his video that you kill the marines in order to make the mission end sooner. For some reason that didn't work for me, so I just took a few extra minutes to rush and finish the level the regular way, making sure that Keyes would stay away from explosions and enemies.
All in all, this is sort of a long mission if done solo, so that's why I recommend finding a good co-op friend to help you. Even if neither of you guys can perform the grenade jump, at least you can kill all of the enemies faster and maybe respawn each other if one dies.MISSION 4 - THE SILENT CARTOGRAPHER
Another kinda easy mission. And another one that I did NOT follow Eli's guide to the letter. I was able to do the warthog slide on that tight little corridor a couple of times. Unfortunately those runs got screwed eventually turned out to become just good practice ones. The next day when my frustration level was low again and I was ready to continue these speed runs, I thought I could save more time and it would be easier just go to the room where you have to activate the button for the door in the main installation to open than if I kept retrying the warthog slide over and over. I finished that level solo in 14:44 minutes, so I think that was good enough.
Back at the main installation, I shot the floor with the magnum a few times to supposedely skip the cutscene, went back towards the door to get the checkpoint just like in the video, and then I started trying to fall down also like in the video. Took me a few more tries to get it right, but I can say now that in fact it does work and it's not that hard to pull off. Then I fell down again, this time over the overshield and kept following the video guide's path and tricks. After activating the cartographer, assassinating the 2 Elites and get the active camo I tried a few different approaches to scape, but they didn't work so good. To avoid that last grenade jump while invisible didn't pay off, so I surrender to it and pull it off after few tries.
I forgot to mention that between the first installation and the second one (main) I got the rocket launcher from the wrecked Pelican ship. If I remember right, you have to kill 4 jackals in order to get near it. That rocket made my life easier on my way out of the 2nd building, when it was time to kill the sword Elite. The rest of the mission was pretty much like the guide.
This seems like a difficult mission, but it isn't too bad. I bet that in co-op certain parts of it can be smooth like a breeze. The most difficult parts are the 2 jumps down and then coming out of the building before the active camo wears off, without being killed by jackals or Elites.MISSION 5 - ASSAULT ON THE CONTROL ROOM
This mission is hard until you can execute the sliding jumps down but the easiest after that.
During the starting cutscene press the "switch weapon" button (in my case, Y) so when you gain control of Chief you already have the magnum. A little better than Eli does in the video, I like to immediately aim towards the door and as soon as it starts openning, I throw 1 or 2 frag grenades to dispatch the 2 elites and most of the grunts. I can pull that off 90% of the time and it sets us with a good start. The first grenade I throw is actually to prevent the Elites from running out and the second one is on the inside. Sometimes if you just throw grenades inside they will rush out or be pushed out by physics.
Then you make your way to the bridge, killing enemies you come accross in the process. No secret in that.
Now, after trying to follow Eli's instructions for the jumps down I wasn't able to reach the ground. No offense, he's a good speed runner, but sometimes I think he talks too much. Forget that 75% stuff that he says. If you take the right side like he does, the only thing you need to know is that you must not get any extra impulse/momentum when falling, so under any circumstances do not jump up before falling down or you will die. Get on the top of that short wall (sometimes a sleeping grunt will spawn there, so silently assassinate him before you can line yourself up) and drop down moving forward, not "75%" but all the way. Do not move to any sides while falling, just stare the mountain wall. Most of the time you will land there without a problem. But there's an easier way to reach that same spot, eliminating the first of three sliding down parts.
I've found on another Youtube guide a guy ("Silver" or Halo Completionist, I'll leave the link below) who instead of falling on the right side of the bridge, chose to jump (and yes, here you can and must literally jump forward) to the left side. You get a much safer landing, the only problem being that it's more likely that the Banshee will see and shoot at you for a while and sometimes an Elite that's hiding on the lower part of the bridge too. But left or right, whichever side you pick, you should come to the same spot for the second part.
Silver's video also does a good job explaining how to do the next slide falling, but it's possible that more often than not you'll get yourself killed until you can get the hang of it. Even then, getting the second jump right and in a short time means that you'll have done everything - including the first jump - right up to this point. It's doable, but it requires a lot of focus and a bit of luck.
For the third and final sliding down part, after a few tries, I went back to Nak3d Eli's video solution. At first it seemed hard to reach the same spot than he did and if I got myself killed after getting there I would be angry for wasting the opportunity and the 2 previous successful jumps. Another thing that made me switch back is that Silver's last slide seems like an accident. He even says "I didn't want to fall down at this point... but I did", so unfortunately he never gets to show exactly where he thinks that the best spot for reaching the ground is. On the other hand, I could eventually find the spot that Eli thinks it's best for the last jump, even if he shows it a bit too fast. Again, it took me a few more tries and frustration handling, but I was very happy when I got it right.
By the way, on levels like this one, the shield bump on Keyes or whenever else you have perform a difficult thing, it's better to restart after a few tries. You don't want to get it right only after 30 minutes trying. What I had in mind was something like "If Eli runs through this level in 7 minutes, and I usually double his time, then I won't go on with the mission if I can't get the last jump right after the clock reaches 7 minutes". So, everytime the clock reached 7 minutes I would restart the mission. The plan must have worked because my time on that mission is 12:39 minutes, solo.MISSION 6 - 343 INDUSTRIES GUILTY SPARK
I don't have much to add about this mission. Eli's video guide is very good here. The only annoying problem I had on my first attempts was that grenade jump when getting out of the installation and escaping the flood. A bunch of things went wrong. One time the little glass in the corner where you throw your grenade was broken, then I ran out of grenades - and you know how this game likes to punish you by giving you a checkpoint after you mess something up. Then I was able to pull off the jump without any grenades (not an easy task, it was an awkward jump), but fell off the second time. To make it short, if you screw it too much like I did, restart. It is a short mission after all.
Another thing that I used to do different than what he shows in his video is that after exiting the installation I just ran straight. That way you will end up where the Sentinels are fighting the Flood and it feels like a shorter path. Once there I used to hide in a narrow corner of the structure and shotgun the enemies only if they came too close to me, otherwise I just let the Sentinels kill the Flood and be killed, until 343 showed up. I did that even on my regular Lone Wolf Legendary run. But for some reason, while attempting for the Goat Roped achievement, the Flood was killing me everytime in that sacred camping spot of mine, so I had to go with Eli and "stay out of the red lights circle". It worked. My time for that mission was 13:22 minutes, solo.MISSION 7 - THE LIBRARY
A few years ago I was afraid of The Library. I was playing Halo CE Anniversary and that's when I started practicing the grenade jumps. That very first grenade jump followed by a crouch jump scared the hell outta me. Now I look back and see that... well, I was learning. Now I have confidence and these things don't scare me anymore. I can do the grenade and crouch jumps on this level much much more frequently and without thinking too much. I still don't think I'm a god on this mission, but I know it well and I'm confortable with it. My time isn't great, but hey, I can do it solo (20:49 minutes) almost as good as I did it in lagging co-op (20:08).
You don't have to be afraid of the frag grenade jumps neither. It is an exact science. You throw it, you get used to how long it takes before it blows up and then just before that you press jump and forward to go wherever you want to go. Where you throw the grenade doesn't matter much neither, doesn't have to be precise, in this case. Eli executes some almost unbelievable jumps having thrown the grenade kinda far of where he wants to jump.
The crouch jump at the beginning of this level are also a simple thing. The more you do it, the easier it gets. Oh, and some people think you need to crouch jump to go around those "wall ramps/bumps" on the circular part of upper levels, but you don't. A regular jump does the job.
Back to the start then, despawning the first enemies on this mission is a must on my opinion. I'd say that it's something expected of you, just like grabbing that first shotgun.
One thing that Eli does (minute 2:13 in his video) that I never did is that grenade jump by the overshield. Even he says that's not necessary, and I don't think that's the safest way to go, specially if you mess up you might end wasting the overshield. Also, just like he says, make sure you don't spend all of your frag grenades. Having at least 1 is necessary to grenade jump some gates when possible.
I also don't care about "making the flood guy dance" (minute 3:30). Move forward and as soon as it says "Hold your position until the monitor returns" I go and crouch by the wall over the ramp. Some "popcorn flood" might come towards you and it's okay to shoot them with the magnum if you see that happening. But stay crouched and shoot as little as possible. As I've said before, I never shoot the little flood spores. If I have enough shields I let them commit suicide.
Two more things about the last part of this level, right before you reach the Index: I prefer not to execute the last grenade gate jump because when I fall into the last room the enemies are spawning so it's really easy to get killed (and waste time that way). After some attempts, I figured that the best way is to get away from the gate (outside the last room), keep going to the right and fight all enemies. A ton of them will come at you but you can easily kill them with grenades, magnum and shotgun. Once things are quiet you can move to the last room. More enemies will start appearing but now you don't have nobody behind you. I usually take a left here instead of going straight to the index, then jump the wall ramp and go for the cutscene.MISSION 8 - TWO BETRAYALS
Another one that's far easier to do in co-op, I think. I think it's a hard mission because so much can go wrong, there are too many Flood with rocket launcher and enemies that can blow you up. When I got good at the first parts I could execute it perfectly and fast, just to keep dying on the room with the 4 Elites or a little after that. That's when I educated myself that taking cover and waiting for the shields slowly come back is faster than dying and redoing.
At the start, with little practice you can dispatch the 4 Sentinels really fast. You don't even have to aim properly, just charge the plasma pistol and let it go. Then, open the door but don't go too much forward or the Sentinels will stop shooting the Covenant and start picking on you. I also like to properly throw a grenade to help them kill the grunts and focus on the Elite. Due to the RNG what happens next tends to vary a bit, so the Elite (might be blue or red) and 2 Jackals around the corner might rush earlier and make your life more difficult or not. If you die, restart.
At the last door before reaching outside, you must press the button twice properly as well, otherwise the game might give you a new checkpoint where all the enemies are spawned. You won't be able to reach the Banshee and will have to restart.
Don't forget to grab the rocket launcher like he does in the video. Once you're ready to despawn all the enemies back you will have to practice the way you fly your Banshee. The enemies need to despawn for you to progress and the only way that will happen is if you fly down close to where they were supposed to be. You have to be fast because as soon as they appear they start destroying your vehicle. If you do this properly you should read/hear Cortana saying "Good. Let's get moving... the Nav Point marks our target". That's your cue to start flying up up to the marked objective. I think that this part could be a lot easier if co-oped. So is the jump on the head of "Bruno", later on.
I think the rest of the video is good. The only thing that gave me trouble was in that part after I got outside with the ghost and no enemies spawned. I couldn't figure what I had to do and wasted that run. The key here is to go slow when crossing the bridge with the Ghost. Even though seems riskier because of the amount of enemies that start popping up, they usually don't harm you right away, so you can go towards the console, go into hidding, activate the door real quick, get back into the Ghost and get the hell outta there. Once you're outside you HAVE to kill the 5 Flood on the right before rushing forward. I think Silver stresses that a bit better in his video. It's good to watch it and get different observations from different people.MISSION 9 - KEYES
Here I was happy and concerned at the same time. Eli does the mission in less than 4 minutes, so I thought I could do it in about 8 (and indeed I could). But I wasn't looking forward to keep retrying the infamous shield bump once I felt how frustrating it was to get it right.
I tried again and again, then I gave up, went to Youtube and started looking for different videos hoping to get a good explanation about how the guys know where the right spot for being launched by the shield is. I watched 4 different videos and the one that helped me the most was another one of Eli's. Again, he talks too much on this video (and I wasn't the only one who thought that 12 minutes is far too long to explain it), so just skip, for instance, to minute 2:05 and freeze that image. In my opinion that's the right position to try it and eventually get it right. Basically your weapon's middle pointer is straight, aligned with the vertical axis of the shield, and the end of the middle pointer is going over the area of the shield. The right pointer's end is touching that other vertical line to the right and the left pointer end is slightly going over that specific purple dot.
You will know that you've done it right when not only you were able to be launched to the floor above, but its ceiling. You might fall off a couple times trying to figure what to do next, therefore having to redo the shield bump. It happen to me 2 or 3 times. Basically you want to jump towards the same diagonal black space that Eli does, after he's done talking. Careful not to jump too much to the middle or you'll fall. Once you're on solid black just jump back the same way he does and you're good. If you bring the shotgun with you then it will be easy to kill the frozen enemies before moving to the Keyes cutscene.
I could have done this mission in 5 minutes if I didn't keep dying after the cutscene with Keyes. I threw the grenade to distracted the guys, but couldn't jump over the Elite fast enough like Eli says in his video and kept dying by shots on my back as I ran right to escape. Then I noticed that as soon he distracts the enemies, jump over the Elite and take a right turn he starts walking backwards with his weapon ready even though there were no enemies behind him in his run. After that, as soon as I turned right, I started walking backwards and immediately threw a grenade to stop any of those enemies that kept killing me from pursuing me. And it worked.MISSION 10 - THE MAW
I'd like to point out only 2 or 3 things about this level:
1- Don't worry about that rocket he gets in classic mode. It might not have enough ammo anyways. Before going into Engineering, go to the Armory (he doesn't do that in the video), quickly grab a brand new Rocket Launcher and ammo, get out and now proceed to the Engineering.
2- When blowing up the 4 fusion reactor things, don't worry if you can't do it as fast as he does in the video. Take your time, play safe. You can get both on one side if after activating the console you go back to the same way you came from. It's important to always go back to the side where you can climb
on those awkward blue and red things, because if you don't you will have to fall to the bottom floor and climb back up, and the Flood enemies will respawn.
3- When driving the warthog, there's no need to do the 360 spin in middle air like he says in the video. All the times I played this level on TMCC I've never had a problem when falling forward with that vehicle.
Link to Silver's video guides:
Link to Nak3d Eli's video guides: refer to the other solution.
M 1 - 8:26 (solo)
M 2 - 19:51 (co-op)
M 3 - 39:45 (co-op)
M 4 - 14:44 (solo)
M 5 - 12:39 (solo)
M 6 - 13:22 (solo)
M 7 - 20:49 (solo) 20:08 (co-op)
M 8 - 29:21 (co-op)
M 9 - 6:59 (solo)
M 10- 13:09 (solo)
Link to calculating/adding time tool (thanks to I Flavius I for that): http://www.grun1.com/utils/timeCalc.html
Now I'm tired of writting, I think I've covered the most important stuff IMO, specially that which don't follow the video guides. If you still have questions, don't hesitate to ask and I'll answer if I can. If any of this end up helping you please hit the thumbs up button to let me know.