The Hangar

You'll begin this level on a catwalk in a hangar.  You'll immediately be faced with several things that you must eliminate.  Once you kill them, look for a broken electrical box nearby and repair it to restore power.

Enter through the unlocked door and use the computer.  Pick up the medical kits in the room and then head out.

Go down the stairs and be ready to be attacked by numerous things, but there are some fuel drums down there that can help you take them out quickly.  You can also back your way up the stairs as you fend off their attack.

Once they are eliminated, go down the stairs and look for a large open room.  At the end of the room you'll find ammo crates, but there are a few things waiting to spring on you before you reach them.  Look for another door near the stairs with a corpse nearby and search the corpse for a note.

Have your engineer repair the electrical box for the crane and a gate will open.  Walk up the beam to the series of beams near the roof.  make your way to the office and you'll notice that it has a hole in its ceiling.  Drop down and go through the hole.

Use the computer in the room and pick up the keycard.  Then exit the room and return to the corpse that had the note - you can now enter the locked door near the corpse.  You'll find a save station there that you may want to use at this point.  Go down the stairs and as you approach another body Collins will reveal himself to be a thing.  Kill him off and then head down the hallway to the boss battle.