Step 1: Keep restarting the game until the box is up at the light house.

Step 2: When the box is at the light house keep using it until you get the scavenger or any other power weapon. But if you use the PAP VR-11 you wil not get the achievement. The scavenger is the best though.

Step 3: When you get the scavenger or any other power gun run around the place where you started and shoot him. I know from killing him it takes less then 36 shots from the scavenger. I think the normal ray gun is about 300 shots and the PAP ray gun is 154.

Step 4: You can tell he is weakening when his light hammer is flashing and when it turns orange he is pretty weak. When it is flashing orange you are very close to killing him.

Step 5: When you kill him you will get one free perk - the death machine or the wonderwaffle and for the last time you will not get the achievement if you kill him with the pack-a-punched VR-11!