This trick works best as a Hunter. Before you spawn in the No Mercy campaign, 
Level 3, go all the way to the very end, at the hospital. Once up the ladder, 
turn to your right, and there should be a dumpster. Move it all the way over to 
the manhole, and one of two things will happen. It will either fall all the way 
down, which is bad because the Survivors can't move it; or it will partially 
fall in. The survivors cannot move this at all. If you are a Tank, you can move 
a car over it, which is better than the dumpster. From here, do not puke on them 
as a Boomer, as it will attract the Horde. The Horde will try to move the object 
away from the manhole. Then, attack them as Hunters and Smokers. They may shoot 
you, but every so often, you can hurt them until finally they die.