Just like the title says! Me and four other friends did it about a year ago. We camped on the left side pf the map. Opened the stairs three people in the front hall way, one wactchem window witha flamethrower. Other thre need ray gun and LMG or HMG Jug, sleight of hand, and quick reive are a must need. Double tap will just waste ammo, bc the higher lvl the less ammo you get, so you need to consever as much ammo as you can. People on the right side of the map need enough points to go to eletric room. Turn it on and then you reunite, get some pretty good gun then go to where the double tap machine is and there ya go. Once you get to about lvl 35, 36, or 37 the zombies barely even come inside, they just stay outside. Its pretty awesome, takes forever but It was fun. Hope I helped you all out who want to do this legitly!