On my PC you dont have to complete anything :P!
Best tactics I found, Stay in the first room till about level 6, then move into the next room but try to keep bording the windows up till it getting abit full on, but all the time be trying the magic box for the ray gun, once you get to like level 12 and you have the raygun move upstairs right at the back were there is only one window and you can concentrate on shooting all the zombies running at you.