Go to the entrance plaza then go upstairs. Go to the end with the bee then go to the door marked "Exit". This hallway is the one from the very beginning that Otis sealed off. It is a dead end but has a lot of zombies that will continuously respawn. This allows you to get a lot of kills quickly. You can also stop by the antique store on the way and pick up a few katanas or battle axes for very fast kills.