Being a Hunter is recommended. Before spawning in No Mercy level 3, go all the way to the very end, at the hospital. Once up the ladder, turn to your right to find a dumpster. Move it all the way over to the manhole and one of two things will happen: tt will either fall all the way down, which is bad because the Survivors can 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 any 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, as Hunters and Smokers, you can just attack them. They may shoot you, but every so often, you can hurt them until finally they die.
In the warehouse near the end of the level, right after the area with the gas station and the lift, there is a forklift which you can use to block the door the survivors must go through. When the level starts, do not spawn. Instead, immediately go to the warehouse. When you get there, spawn and start pushing the forklift to the door. To push the forklift, repeatedly attack it. It will move slowly but you will have time to push it before the Survivors reach it. The only way the Survivors can move it is by throwing pipe bombs at it repeatedly.