the easiest way i found was to go for this on "FIVE". i was with 2 others and we held off in the start room for 5 waves without buying a weapon. NOTE: make sure as others have said use pistol then knife to finish the zombies off, wave 6 open door to elevator/lift and buy machine gun off the wall hold out till you have around 6-7 thousand points then find the box and try your luck. if you run out of money then i suggest going back to first floor and hold out by the lift 2 guard the windows and 1 guard the hallway and switch so you all get decent points. make a crawler then try your luck again.if you get the crossbow activate the power and go back up to floor 2 and pull the switches to get to defcon 5, enter the room to pack the punch it and return to hallway on 1st floor where you shoot the person guarding the hallway with crossbow.NOTE: it will be around wave 11 and the thief comes, so take out your other weapon so he takes that instead of your crossbow.