at about 20 goback inside and open all the doors up then die, about 5 waves later go back in and the zombies cant get in and you just use the ramdom box, only send one person in to check and see if it has worked, then send another in, hint 1-2 are together 3-4 are together so if 1 dies he will respawn by 2, if 3 dies he will spawn by 4 so only send 1 out of the couples in, this can vary from different games and you may have to wait till wave 30, so the zombies cant get in. i recogmend you do thr gun glitch with betty's
hope this helped