Instructions
Walk up the stairs in the room you start out in -- go as far as you can without clearing the debris. Zombies will be able to enter from the top of the steps and you will not be able to reach the ledge if you clear the debris.
Turn around and find the ledge that sticks out partially from the ceiling. Run forward and jump onto it -- this may take a few tries.
Stay on the ledge and kill zombies as they come up the stairs -- they will not be able to touch you, so you can stay on the ledge as long as you want and accumulate as many points as you want.