The Pillar of Autumn Walkthrough
From Halopedia, the Halo wiki
Part 01: Reveille
As soon as the first cutscene is over, you should see yourself climbing out of the cryotube. A flustered Thom Shephard will lead you out of the cryobay, but will soon be killed by a door exploding as it opens. Double back, you'll see a pair of pipes on your right (if you're facing away from the partially opened door), jump over it. Move through two more doors, and crouch under a half-raised blast hatch and move forward. Ignore the firefights going on in the hallway, you'll have plenty of Covenant to kill once you have a weapon, and there's nothing you can do to save these Marines and techs anyway. Move to your right, and you'll see a second door on the left side of the corridor. Move through a darkened room, follow the hallway, and you'll soon see a pair of techs and a Marine firing on some Covenant. The Marine will offer to take you to the bridge, you can follow him or cut around and hurry ahead yourself. Once you reach the bridge, approach Captain Keyes near the viewport, and a cutscene will start.
Part 02: AI Constructs and Cyborgs First!
Once the cutscene is over, Keyes will hand you a pistol. If you're playing single player, here's a handy trick to get you through this level on Legendary in one piece.
As soon as you can move, get out of the bridge as quickly as you can, taking care not to run into a wall or any of the bridge crew. If you're fast enough, your pistol will not appear when the door opens into the cafeteria.Take cover behind the closest steel barrier. Shoot down the Grunts using your pistol (Aim higher on the Grunts-they usually will jump after the first shot and it will take two pistol shots to mow down a Grunt on legendary.) they're relatively easier to kill and will cut down the Covenant numbers faster. If you are fast enough to kill them before they throw plasma grenades, you will get enough grenades to stick the Elites. If you come under heavy fire, crouch behind the barrier. It's most likely a Marine or two has already been killed, so make sure you appropriate their ammo and grenades. Grab a Plasma Pistol or Rifle, they're very effective against Elite shields. And make sure you obtain plasma grenades too, they can kill enemies very easily on this difficulty.
Once the room is cleared, swap your undoubtedly nearly-depleted plasma weapon for a fresh one, there should be plenty lying around. Return momentarily to the bridge, make a loop around the room, and go back outside. This time, your pistol will appear, along with a trio of Grunts. You now have three weapons to keep your chin up in any sticky situation: A rifle that can cut down a lot of unshielded enemies at once with its rapid fire, a pistol that can kill enemies quickly with headshots, and a plasma weapon that will bring down shields in a matter of seconds. If you're playing cooperative, or if you haven't managed to get all three weapons, I'd recommend an Assault Rifle and a plasma weapon. There's always going to be more ammo for them on this level, and on Legendary, every shot counts. Whenever your plasma weapon drops below 30% ammo, remember to get a new one. Use the doorways for cover. Health packs are all around, so use them. Don't waste them, though, using them while you still have 5-6 bars of health is totally a waste.
The room outside the cafeteria should have a squad of Grunts, and occasionally an Elite. Take down the little guys with a UNSC weapon, and if there's an Elite, knock out his shields with your plasma gun, and switch to your human gun to finish him off. There's a rack with extra ammo next to a health pack, if you need either. Proceed down the hallway to your right, and you'll see an Elite being attacked by three Marines. If you can, go up behind and melee him in the back. If he spots you, stick him with a plasma grenade and back off. Once you leave the hall, you'll see two more Marines guarding an airlock, which will explode and kill them. Toss in a couple of grenades through the open hatch, you should be able to take out a lot of Grunts and maybe some Elites while they're still stuck in the narrow space, and it'll make the ensuing fight a lot less hazardous. Pick up an overshield from inside the boarding craft, move on. There's a Grunt standing in a darkened hallway, use your flashlight to see him. Take him down, and keep going.
You should see three Marines fighting some Covenant from behind steel barriers. Don't go to them yet, move right to a perpendicular hallway that can let you put some flanking fire on them. Again, throwing in some grenades will increase your advantage. Once the first group is taken down, get to the Marines, and wait for the second group behind the barriers. Once they come, throw more grenades, and kill the survivors with whatever gun is suitable. There's more ammo and another health pack near the next hallway. Once you see a tech killed by an explosion and smashing into a wall, move towards him, and once again, flank the Covenant. They're in a perfect spot not to see you stick them with a plasma grenade. Once they're dead, proceed to another airlock, there's more overshields if yours ran out. Take down the enemies, starting with the Grunts.
You should see another half-raised blast door, move under it. A pair of techs will be running from Covenant ahead, but you probably won't be able to save them. Don't run towards them, let the Covenant come out so you can take them down. Use the lockers and benches as cover if you need to, there's a lot of enemies in the room. If there are any left, run to the right side and kill the Elite first. Turn around, and kill the others on the opposite end.