Lone Wolf Legend - 90G (Original Achievement)
To get this Achievement, you must complete the entire Campaign on Legendary difficulty while playing alone.
There is a big glitch in this Achievement at the moment, and its a positive one.
If you have beaten the game once before, you can now select whichever mission you want, set it to Legendary, and go to the last Rally Point in that mission.
This will skip over half of the mission and it will still count as completing the level on Legendary. Of course this means you will still have to complete that small portion on legendary, but it's much easier than the whole thing.
1. The Noob Combo is still MASSIVELY helpful against Elites. Using the Plasma Pistol at full charge to take out their shields, and then quickly switching to a precision weapon to get the head shot and finish them off is highly recommended. Be careful though, some Elites can be really mad, causing them to pull out their Energy Swords and charge you.
2. Jackals. These guys are usually covering themselves with a shield. You will notice a small hole in the left hand side, if you are accurate you can shoot them there with a precision weapon to make them lower their shield, and then get the head shot with the second shot.
3. Grunts. A simple head shot with a precision weapon can wipe these out in seconds.
4. Hunters. There are a few points in the game where you will have to deal with these walking tanks. Always remember they travel in pairs so if you see one, there must be another nearby. The best way to get at them is by throwing Plasma Grenades and sticking them, bombing them with Rockets/Fuel Rod Cannon, Sticky Detonator, or the Shotgun/Scattershot. You should be able to find one of these weapons near each set of hunters you come across.
5. Crawlers. These annoying enemies are very fast and can use multiple weapons. A single head shot can destroy them instantly though. Be careful as one or two in the game tend to have the Binary Rifle which can instant kill you.
6. Watchers. If you see these with flying over other Promethean's, I suggest killing them first as they will bring Knights back to life, and on Legendary, you don't want that. I suggest assaulting them with Precision weapons.
7. Knights. These are the real enemy in the Legendary run. They are best take out with close range Shattershots, Needlers, Incendiary Cannons, or by shooting them with a Precision weapon from afar. The precision weapons will take a toll on your ammo to kill a Knight but its worth it for each kill, just make sure there are no Watchers in the area before fighting. Knights tend to teleport to another location when under fire so keep yourself in cover. Knights also like to fling grenades at you and these can instant kill too if you stay inside their sphere. Knights can hold Binary Rifles and in the last mission Incendiary Cannons, so be very cautious.
8. Know the terrain, you really need to know where your going and what fights you can skip all together by running straight past them.
9. Don't rush, take your time and move slowly, if you come across an enemy sniper, you will die instantly if you aren't careful.
10. Vehicles. When using vehicles on Legendary you need to be careful. Ghosts are usually instant death, so don't use them. Warthogs can be useful with Marines as gunners (they seem to be good shots in this game). The Scorpion/Wraith tanks are very helpful in clearing out some areas of the game and are highly recommended to fight as far from the enemy as possible. The Mantis is a must when you find one. Never get close to the enemy and always keep an eye on the skies for Banshees, unload everything you have and stay in cover when you can. Banshees are your best friends, switch to the fuel rod cannon with the Y button and bomb everything that moves. Just don't get too close to the ground as an Elite may try to skyjack you or an enemy with a plasma pistol can take you out (usually into a bottomless pit). Lastly as said above, Plasma Pistols can paralyze ANY vehicle for a shot time, this can be massively useful against Ghosts as you can quickly stun them, run up to them, hijack them, and use them to kill their driver.
Lastly I suggest turning on the Grunt Birthday Party skull, this way you can tell if you are getting head shots on grunts. Its not essential, but it helps.