DISCLAIMER: This guide is intended for people who don't have a dedicated group of 6 to play with (or just don't want to do the whole raid, I suppose) and I'm going to focus on the Vault of Glass for those without the DLC (like myself). The general premise works for the Crota's End raid (which people tell me is easier in general) but I don't have the experience with it to write an in depth guide. If you do feel free to share in the comments.
How to prep yourself: lxl Linc lxl covered this pretty well so credit where credit is due to him but I'll update things a little with new developments. How to unlock hard raid hasn't changed, beat it on normal first.
Next up, be level 30+. This is a LOT easier now that the level cap has been raised to 32. You can buy legendary gear from the vanguards/crucible/various factions that will let you get to 31 in combination with an exotic armor piece. Buy those and upgrade them. This will take time, but farming raid, public events, daily story, etc will help get the ascendant shards faster.
Oh yeah, and have a mic, communication is key.
Now you're ready. Let the guide begin.
This website is your friend, it's full of people looking for raid parties you can message about joining:
destinylfg.net
If you look around you'll notice that you can sort out the postings by what event you want to do. I'm going to suggest you set that to Raids-Vault of Glass-Hard-Atheon. Why? Because you don't have to complete the entire raid to get this achievement, as long as you kill Atheon you'll get it. The postings are pretty active so you shouldn't have a huge problem finding people who already have the checkpoint at Atheon unless you're playing at weird hours. Just send the posters a message with your class, level, gamertag, and any other pertinent info they might want to know and let them invite you to their fireteam. The site is pretty busy so you may need to message a few groups to find one that still has open spots, just keep at it.
Now you're in so I'll give you my guide to killing Atheon.
Basic Rundown of what's going to happen: Same as the normal raid for the most part. Atheon will appear and send 3 guardians at random to either Mars or Venus to grab the relic and kill oracles while the other three open up the portal for their return, HOWEVER, on hard the people who don't get sent away will be put in a stasis bubble they must shoot their way out of (similar to what happens in the templar fight). PRO TIP: don't use rockets to shoot your way out, you'll kill yourself and won't be able to respawn on hard. Once everyone is out of the portal collapse to the middle to unload on atheon while to relic holder protects you from atheon. Wash, rinse, repeat until he's dead. I've done it in two iterations of this before, not sure if you can do it in one.
IN DEPTH GUIDE
Now I'll go through what you should be doing at each step along the way and in each role. Basic Relic info first:
If you don't have experience with the relic don't take it unless you have to. That said, if you have it the important thing to know is you need to kill the enemies where you get teleported. I find doing a ground pound the best (jump + RB when in air). From there you need to escort the two killing oracles while using LB to cleanse them so they can see. Once through the portal run to the island in the middle and just hold LB while everyone fires at Atheon.
TELEPORTATION-Mars
In my opinion, this is what you want to happen because I find Mars easier to deal with than Venus. CALL OUT WHERE YOU GOT TELEPORTED IMMEDIATELY, it's just plain easier this way. The relic holder needs to run right at the 3 hobgoblins in the center. Usually 2 are close enough to kill in one ground pound, then repeat on the remaining one. Then run back to the two teleported with you to cleanse them repeatedly. If you're killing oracles do your best to stay with the other guy doing it with you (AKA your battle buddy) as you slowly move towards the gate, it's a lot easier for the relic handler to cleanse the two of you if you're together. OF INTEREST: Golden gun DOES WORK on the oracles, a hunter with Achlyophage Symbiote is a huge plus for this part. It may be best for the relic holder to head out the gate first so he can set up his LB in the island in the middle for everyone to collapse to but it's not totally necessary. When you go through the portal the center island will be on your left (this is useful since you can lose sense of your orientation going through the portal)
I know what you're thinking now: "But LethargicVirus0, what do I do if I don't get teleported?" Never fear, I gotchu bro. Start by shooting your containment shield off you (not with rockets!!!) and help anyone else still dealing with theirs when you're done. Then go to the sync plate associated with the planet you're teammate is on (from where you spawn Mars in on the left and Venus is on the right). For Mars I would have one guy stand on the pillar in the sync plate and the others on the larger pillar between the sync plate and the portal, just make sure someone is on the plate keeping the portal open. From here just kill the supplicants, watch out as they'll explode when they die and can kill you. Once they've been killing oracles for a while call out to your teammates which way they need to turn when they come out (for Mars it's left). This is helpful because you can visualize where they'll be coming through better than they will. Once they're through get to the middle island and inside the relic's cleanse shield.
TELEPORTATION-Venus
The same basic rules apply from the Mars teleportation but there's a few other tricks to this. First, the relic holder will likely need help taking out the minotaur that spawns on Venus. He's usually on the left side so just throw a grenade down there if you're on Venus and not carrying the relic. Relic holder: go for the minotaur first since he'll kill you the fastest if left alive, then kill the goblins. Then proceed on just like on Mars except this time when you go through the portal you'll need to turn right to get to the center island.
Now, if you don't get teleported your job is a little bit more complex than for Mars. It starts the same, get rid of the containment and open the portal. The spacing I like for this is two on the pillar in the sync plate for Venus and one on the taller pillar over by the Mars portal. There's two reasons for this: 1) the supplicants swarm around the Venus sync plate more than the mars so keep someone on the other side to shoot them away while not at a large risk of getting blown up and 2) sometimes the people who get teleported need help killing the oracles in there. Once the portal is open one of the two on the Venus pillar can run in the portal and shoot the oracles as well. Problem with this is once you go in you will start to go blind and won't be in a good position to get cleansed so I advise going in, shooting what you can, go back out while you can still see enough and jump to the center island and let the relic holder make his way to you since he's going there eventually. Going in too early can leave you blind for a while but going in too late can mean missing an oracle and everyone dying, use your best judgement on this one. Don't forget to call out that they need to turn right out of the portal when they return!!
THE (NOT-SO-)LONELY ISLAND
Once you're done with the oracles everyone needs to get to the center island and the relic holder needs to hold LB down the whole time you are there (preferably on the front half of the island). Be sure to stay in his cleanse shield because Atheon will kill you super fast if you leave. The main idea here is Atheon is more susceptible to damage now and you can unload on him with your guns (Ice Breakers are a good idea here). There's a few do's and don'ts I have for this part. I'll start with the don'ts: DO NOT USE ROCKETS!!! Seriously, if you've read this far you should realize that rockets are a bad idea, you're just asking to get yourself blown up. #MachineGunMasterRace. Also, here you'll have ridiculously decreased cooldowns on your super/grenades/melee but I wouldn't use them (aside from defenders but more on that later) since they tend to push Atheon around and can make it harder to hit his crit point. That's about it other than don't leave the shield.
Now for the do's: Titan defender's are a plus here, just equip the Weapons of Light perk for your Ward of Dawn and place it overlapping the back of the cleanse shield like a Venn Diagram (this is why I said the relic holder should put the shield on the front half of the island). Then everyone can just pop in and out and get even more bonus damage on Atheon. Once you have that just unload your guns at his crit point (stomach) while watching the timer on the left. Take responsibility and call out to your teammates when it gets down to 5 seconds or so because from here you have to do a few things.
When that timer hits 0 the relic is going to disappear and with that goes the relic shield so it's on you to avoid Atheon's blasts and there's two main ways to do this. The basic method is for everyone to bail to the sides (towards the sync plates) while the shield is still up except the relic holder who will slowly back up to the back half of the island and then jump off when the relic disappears. If you're going to do this make sure to do it with a few seconds left on the timer because if you're still jumping when Atheon teleports your fireteam again you can get put in containment over the gap and fall to your death.
Now the other method requires at least one titan defender. With 2-3 seconds left he will put up his Ward of Dawn around everyone so that Atheon's blasts will reflect off and everyone is safe. The downside to this method is that being in a Ward of Dawn can make it harder for you to shoot the containment shields on you since you can't shoot out which can delay you opening the portals. I'm throwing this out there because it is an option, not necessarily because I think it's the best option (I do think it's great for normal where there isn't containment shields though).
From here on it's really just repeating the above steps. If you have a full fireteam doing things right it should only take 2-3 teleports to kill Atheon. Use your best judgement, if you think you can use the last few seconds of having the relic to finish Atheon rather than set up for the next teleportation phase do it, just be aware of the risks. The only other thing I haven't really mentioned yet is to pay attention when jumping to and from the island since if you miss you'll die and look like a fool. It sounds silly but it happens for a couple reasons, 1) everyone is going to the same place and it's easy to bump each other off and 2) some people will be partially blind from the oracles so you might not realize you're about to bump into someone.
As with other achievements in this game, it may not pop right when you beat Atheon. Don't worry, you don't have to beat the whole raid on hard from beginning to end. Things you can do to help it pop include waiting (normally a day or two later it will pop when you start up the game) and going to the change character menu and instead of selecting that choose to switch profile. When you log back in it can pop when selecting your guardian. For me personally I switched profiles immediately and it didn't work but when I did it again the next day it popped.
And that's it for the guide folks. I'll update this if I come up with any more tips and tricks while playing or if you guys comment any of your own I'll add them too.