Try to stick to the left path as best as you can (don't go down stairs yet) and you'll come to a garage with a partially opened door. Inside the building, you'll find two bodies on the floor under a Crimson Omen mark. Between them will be [COG Tags]. You can now head back and go down the stairs.
Further on, you'll come to a point where Paduk is talking about how the generator failed. If you go over and reactivate it, you'll be ambushed by Fallen. These act like a faster, less explosive version of Wretches so keep moving and fire from the hip. This will soak up a lot of ammo but keep shooting and eventually they'll stop coming. Go through the newly opened side door and inside, wait for Paduk to open the gates.
Start making your rway through the ruined building. Keep moving but watch the doors on your left because eventually when you pass one, more Fallen will jump through. As you continue, more and more Fallen will come at you but you should have plenty of room to back up if you can keep track of the bends and turns in the path.
When you finally get back outside, stock up on ammo before heading up the stairs. At the top, you can take an elevator up to the Imulsion Rig.
