On the 4th level of no mercy, as the infected, quickly get to the top of the elevator shaft and go outside to the roof.

You will see a yellow generator, as the infected start hitting the generator until you can position it in front of the elevator doors.

This is bad for the survivors because the cannot move the generator.

Note: do not vomit on the survivors as the boomer because normal zombies can move the generator.

Another note: this works best with the hunter because he can move the generator faster than the other boss zombies but you'll have to hurry because the survivors will try to get through as fast as they can.