For the 360, there is a very simple way to make sure you win on the level where you have to call the elevator on No Mercy. At the beginning of the match, make sure you are either a Hunter or smoker, (die quickly if your a boomer) and then go through the hospital until you reach the top of the elevator where the survivors exit. Looking from the door of the elevator, you should see a yellow thing attached to a small black cord. Get behind it, and start pushing it by clawing and moving it with your body. Make sure to get a good balance between pushing left and right so that you can push it in front of the elevator door. Put it as close to the door as you possibly can. If done right, when the door opens for the survivors at the top floor, they well be mad as hell, as all you have to do is slowly peck off their life with boomers, hunters, and smokers.
IMPORTANT NOTICE: Only 1 person should work on the glitch, the other 3 are needed to keep the survivors occupied.