Find the room with the pentagram floor, lots of rockets, and the invulnerability and stand in the center of it, facing the only way in or out.

Take the jumppad out, go to the right. Jump the little hole in the floor, and you'll be in a room with a yellow armor and some ammo. To your left will be a jumppad.

Take the pad, get the chaingun (if you're doom/strogg). Take the first right avaliable to you. It will end in a hole in the floor. The shaft drops down quite a ways to a jumppad.

However, there's a little hole in the side of the shaft that contains the BFG.