Once you get to Larry's truck it is almost time to do this achievement. Before you continue buy some stero's, mines, and turrets. Proceed on into the courtyard and you will have a monsterfest to deal with. Compete this and hurry and place a turrent. This fend off against the monsters that spawn right after this fest. Then move fast twords the bleachers. Destroy the old turret and place another one a couple inches in front of the entry way behind the bleachers to keep enemies from entering. Then take the janater to the door and let him do his stuff. In the mean time place a stero infront of the door. Now take out your Minigun and keep watch and kill any enemies that get through.