Under the stairs you’ll find some pick ups and a tunnel which you can crouch and enter. Crawl along and grab the medkit on your left and the ammo on your right then you’ll reach a room with a zombie in it. Go to the hatch and when it opens climb down the ladder, there are some trites at the bottom. Grab the air canisters and the medkit and move quickly through this section. You’ll find a lift on your right, hit the call button and enter when it arrives.
When you reach the top there’s a zombie and a z-sec waiting and an imp will spring out of an alcove on your left, grab the pick ups in the alcoves left and right and when you move into the chamber look up behind you to see another imp. Climb the ladder on the right and take him out and grab the armor.
In the next chamber there’s a z-sec to your left when you enter, crouch under the jammed door on your right and grab the pick ups. Back in the chamber head up the stairs on the left and watch out for two cacodemons who spawn in.