First of all, you are going to play solo. Knife the entire 1st round and then open the top door. The mystery box should be in there. If it is, throw a grenade under it, then walk away. Wait for it to blow then go to it but have your back facing the box. Buy it, and stay looking away! When it says trade weapons,buy with your back facing it! By doing this, you get a Python, Thundergun, Commando, and AUG.