This is an extremely cool trick to get down the first bridges in Assault on the Control Room. If this is done correctly, then there will be no more enemies left in the level. The downside is that you can't finish the level because there is a locked door in the way.
Start the Campaign level Assault on the Control Room in Co-op mode. Get to the part were Cortana starts talking about the weather patterns, and there will be two bridges. First, kill the sleeping Grunts with a melee attack. Look to the left side of the bridge; if you look over the side, you will notice that the rock wall has a ledge on it.
Next, go to the left of the door. Stand against the wall facing the edge of the bridge. Then run forward and jump over the edge and land on the rock wall ledge. Have the other player do the same. If you lose any health in the fall, kill each other so that you respawn with full health. One person stays right there and the other person goes up a bit forward, not too far.
Then the guy who did not move has to fall off the ledge. When he starts to fall, the other guy has to start running forward on the ledge. At the end of the ledge there is a little hill; keep running straight. The guy who fell should die right as the other guy is running up and off the little hill. When you start to fall, keep the left thumbstick pushed forward as if you were running. Two and a half seconds before you hit, push the stick to the left. This is the most important part. If you did it right, you should slide of the white ledge without dying and die on the ground below. The cool part is that your partner will immediately respawn as you slide off.
Now it gets easy. Wait for the other guy to respawn. Then, go face the people on the ground below, turn right and go along the white ledge until you see that the edge of the ground goes down and then up and it forms a divot. Stand in the divot and go off really slowly. After you start to fall, fuci back and hug up against the cliff side. You are aiming for a white bump at the bottom of the hill. If you do everything right, then you should live through the final drop. Have the other guy fall because he will respawn.
There you go. Now the entire level will have no more enemies. Have fun!
It is very hard but possible to get down with one player, though again, very hard.