Start by heading forward through the rooms towards the objective marker. There's some hologram powers by the door in front of you.
Emerge into the hangar bay to find the Covenant attacking the scientists. You'll have to deal with the usual: Grunts, Jackals, Elites, and a couple snipers.
Proceed forward to almost immediately be attacked by more Grunts, Elites, and snipers. This is pretty much how this mission is going to go: Constant attacks from Covenant forces everywhere you go. Be ready for it.
Head through the corridors and up the stairs. There's a DMR to your left which can come in handy here.
As soon as you enter the hangar, you'll notice the vast amount of Covenant here. Phantoms will continue to drop off Covenant forces until you seal off the hangar doors.
There's a turret to your right that can help immensely with clearing them out.
As soon as Cortana tells you to hit the switch, book it as fast as you can to the console and seal the doors to stop the attacks! After they're closed, mop up the last of the Covenant forces and head to the objective marker.
When you open the marked door, an Elite and two Jackals will attack you. Assault rifle is the best option here.
More Covenant forces attacking friendlies here. Like I said, this is how the mission is going to go the entire time.
Keep heading up stairs and follow the objective markers, dealing with constant Covenant attacks along the way. Trust me, there's too many to recount here.
When you head through the marked door, take a right and follow the path.
You'll be told a pair of Hunters forced their way in. To assist, there will be a thruster pack powerup, and a shotgun. Definitely take the shotgun, but it's up to you whether you want the thruster pack.
Try your best to shoot the backs of the Hunters at close range with the shotgun to take them down as quick as possible in these tight quarters. Grenades help a lot too.
When they're down, open the door and meet with Dr. Tillson.
After the cutscene, head through the door on your right and follow the corridors.
You'll end up at the Composer. Head across the room to the objective marker given to you and through the door.
Wait for the officer to open the door for you, then head through and take down the Grunt and Jackals waiting for you.
Head through the service tunnel lit up with red. Sneak up behind this dude and assassinate him. Gotta love when they set it up nicely for you.
When you emerge from the tunnel, Covenant will be waiting for you, including one on a turret. If you can, try saving your shotgun for the Elites, it works beautifully if you can get them close enough to you. When they're gone, head forward through another door.
Another day, another Covenant ambush. Tons of Grunts, Jackals, and Elites will emerge. Not like it's gonna stop the Chief anytime soon. When they're gone, head to the objective marker and follow the corridors again.
Creep up behind the unsuspecting Elite here and assassinate him. Again: Love it when they set it up nicely like that. Be careful of the other Jackals and Elite in here also.
In the room where the scientists are hiding, you can find some weapons and an autosentry powerup if you want. Keep moving forward.
Yeah this is getting ridiculous now. When they're all finally dead, head up the stairs toward the objective marker. Again. But you probably figured that out a long time ago.
When you get to this room, watch out! There are two Elites in here, one of which is cloaked. If you still have your shotgun, it shouldn't be too much of a problem. When they're dead, insert Cortana into the console, then remove her when she's done fiddling around.
Continue on, fighting your way through more Covenant. Take a left at the end of this hallway.
I shouldn't even have to say anything at this point. Kill them, move on, rinse, lather, repeat.
Here's where things get real. You have to defend the Composer from an army of Covenant soldiers. Grunts, Jackals, Elites, Banshees, Ghosts, Wraiths, you name it, we got it!
Lucky for you, there's a Mantis here to help out. Get in it and fight for your life against the oncoming waves.
All done? I'd assume so since you're reading this now. Take a breather, you earned it. Then head towards the elevator marked on your HUD.
Start up the elevator and...
Huh. Well then. That just happened.
When the elevator reaches its floor, hit the button to get out.
That's it, time for the final mission in the game!