This needs to be in co-op or Xbox Live.

1. By the stairs in main room get in the corner in front of the grenades (works best with the glitch nazi zombies invisible wall).

2. Have your friend stand in front of the pillar near the closest window to the help room (not by the help room instead by both the stairs, if you're by the help room it makes it harder).

If done correctly the zombies should be running half way to you, then running back up the stairs and then back down the stairs (etc).

Note: if you dont want to waste your ammo then just keep throwing grenades.

Warnings: if you or your friend move too far away then the zombies will rush the closest person.