The Armory!

Hello there, I'm having some trouble in completing the armory-level.
I know I have to get a keycard somewhere down the armory to access the elevator to the surface. Only, the game is so damn hard at this point (and it's in easy-mode!!!!!!).
When you get to the point where a medic is being chased by three armed men (one of them with a flamethrower). Sometimes the medic doesn't change into a thing, but if he does, it's impossible to decent the shaft without being killed.
The trouble arises when you have almost reached the door below.
When you decent the shaft with the medic you've saved, you have to watch the steampipes, the automatic wallmachineguns and the little things that come out of the bodies that are in amost every corner of the stairs.
The point where it gets really difficult is when you can see the door down below. There are two bodies over there and little things keep coming out of them no matter how many you kill. When you try to reach the door while running towards it, a big fourlegged thing appears. The combination of these little things and the fourlegged thing is just terrible. I've only been through the door one time, but I was killed by a wallmachinegun that was behind it. How in the hell can I complete this level in an easy way?!!!!!!!!!