And you though Control was tough! This is the toughest level in the game. But, this guide should help.
Sidestep out of the begining alcove so that you're immediately facing the right wall. Shoot the guard (at least try to injure him) and then move around the pillar so that the other two guards don't kill you. If the first guard isn't dead, angle around and shoot him again, being careful to avoid the other two. The other two will often kill themselves with a grenade, but if they don't, cautiously peek around and try to pick them off without being seen; you'll need a lot of life later (at least you will the careless way I play). This next part can be exceedingly difficult and is often the place where most players lose most of their life: Go through the door very craefully and slowly, edging your way forward until you can see any guard. Once you see part of someone, zoom in with your AR-33 and pick them off. Coninue to do this as you edge forward until all guards in this room are dead (and always be extra cautious when you hear the sound of a grenade landing). Once this room is clear, make your way across the birdge and go through the door at the far end of this room. Run in and then out of the third room very quickly, just so that two guards will see and come to you. Shoot them when they get to the doorway. Go back inside the third room and hide in one of the side chamber areas. You should be able to see at least one of the three remaining guards from here; use the edging-along-the-wall method and take out all three. Next, open the door (the one that looks like a maiframe) by pressing 'B' on the computer console in the chamber next to the door itself and kill the sentry by aiming at his head and shooting once. Do this to all the guards in the next room, too, so that the others won't hear you and attack. Once all the guards are dead (including the two in the exhaust bay), shoot the computer on the left side of the exhaust bay. Go through the vent behind it and wait close to the doorway near the auto machine guns. Wait for a while and 2-4 guards will eventually come to the doorway to attack you. Kill them, and then destroy the drone guns on either side of you. Turn right and follow the vent until you see the body armor. Get the body armor and then turn around. Go to the end of the vent system (where there's another metal grate). Stand just close enough to the metal grate that you can see a drone gun on the opposite side of the room without being hit, take out the drone gun, then get practically outside of the vent and take out the drone gun that's almost right next to you. Quickly duck and move behind the boxes on your left. Scoot back a little and from here you should be able to see the two remaining guns safely. Destroy them, occasionally looking at the metal grate on your left to make sure that no one is trying to sneak up on you. Once that's taken care of, CAREFULLY kill the 4 or 5 enemies hiding behind the metal boxes in the back (the easiest way to do this is to zoom in, then sidestep just enough to see part of one guard and then repeating this method until they're all dead). Now turn around. You should see a ladder behind you. Climb up the ladder and go to the end of the platform. Push 'B' while standing next to the computer to close the exhaust bay and go back down the ladder. Go through the grate on your left, killing any enemies. Go down the first set of stairs, and then the second set on the right wall. There's a doorway up ahead, so carefully go through it just far enough that you can shoot Jaws and get his attention. You can kill Jaws by just getting right up next to him and shooting repeatedly. His arms stretch out so far the he can't hit you. You'll still need to watch out for guards coming at you from almost any direction, but this is the easiest way to kill him. Once he's dead, pick up his smart card and run back through both grates, through the ventillation system, through the exhaust bay and all the way back into the room that has all of the monitors in it. There should be an area blocked off by glass at the front of this room. Kill any immediately present enemis, then go into this glass-enclosed room and shut the door behind you to slow down any enemy attacks. Upload Q's data into the disk drive on the left table and pick up the DAT on the other table. Now run as fast as you can back into the room with the shuttle in it. If you went fast enough, you shouldn't encounter any (or at least many) guards yet. Go to the mainframe in the back and upload the DAT. Now run back to the ladder, killing any enemies in your way. Climb up the ladder, go to the computer and open the exhaust bay doors. Now just stand your ground and kill any enemies that climb up to your position. Mission accomplished.