Make sure you're doing co-op mode, and select "Assault on the Control Room". Get to the area where there is a two-floored bridge, and drop to the floor below. A pelican dropship should be zooming overhead; from the below floor, head in the direction of the door you came in through. You'll see an archway that connects the bridge to the cliffside; both of you should jump onto the archway, then onto a small, rocky ledge below that one. While standing on this narrow ledge, walk down to the lowest point possible, then look downwards; you'll notice a white ledge far below - that is your goal. Have your teammate just jump off and kill himself. Immediately after that person dies, slide down the cliff wall and duck before landing on the white ledge. You should slide off and die, but your teammate will re-spawn on it, and so will you! From here, walk right until you are just above a huge pile of rocks. Try sliding down the cliff wall and landing on the rocks. If you make it safely, have your teammate kill himself so that he re-spawns next to you. Now, there are no enemies for the rest of the entire level! If you backtrack to the two-floored bridge and go into the entrance door, they'll come, but as long as you don't, you can now complete the whole level without wasting a single bullet! It is possible to reach the white ledge without the help of a teammate, but it's very difficult.
Submitted by: Clark Beyer