In No Mercy, Level 5, when you climb to the rooftop, there is a small building with a satellite dish by where the helicopter lands. Climb on top of this building. There is a small staircase on one of the sides leading up to a sealed door (only zombies can open it). Stand above it on the edge, and knock the zombies down as they come up. Eventually one will knock the door down. The Survivors can stand against the back wall in the room, and no infected will come in, including the Tank. Once the helicopter lands, throw a pipe bomb, and quickly run to the helicopter.