Automatic Ray Gun from mystery box on Nacht der Untoten
First, open the help door once you have enough points to buy a Thompson off the wall. Buy it, then make a crawler with a grenade. There are four or five army helmets scattered around the first floor. Shoot all of the army helmets with the Thompson (also use your knife) into the mystery box, then buy a gun and the Ray Gun will appear. Note: This takes some time.