During versus on the campaign "No Mercy" when the survivors go through the sewers quickly run all the way to the exit of the sewers and turn right, you will see a green dumpster.

Start hitting the dumpster until you get it over the manhole, the survivors will get stuck because they cannot move the dumpster because they are on a ladder and cannot hit.

After you do this just sit back, relax, and let your partners work on the survivors till they die.