Just adding a text walkthrough in case anybody prefers those or doesn’t have time/patience to sit through KriSpy CharMZ’s video that he posted.
What you’ll want to do is open power and activate pack a punch to spawn the Megaton (big radioactive dude). When you kill him, he will split into two smaller radioactive enemies. When you kill the second one, pay attention to where he died because he will drop a keycard. Pick up the keycard and run down to the Weapons Lab (underground in a room to go through to get to pack a punch).
Insert the keycard into the slot, and a drawer will open in the cabinet. Pick up the machine, and run back to the start of the map and interact with the barred room inside of the Nacht building. Go over to the hole in the wall and hold X to activate the trap. This will insert the machine into the trap, which activates it.
The next part isn’t necessarily trick, it can just be a bit of a pain. What you’ll want to do now is train zombies (I did it in the yard cause it was right there) until you bring them back into the building and activate the trap. This will begin killing zombies and sucking their souls through the eyehole at the top of the door. IT WONT KILL EVERY ZOMBIE, SO BE CAREFUL.
You’ll know once this is done when the speaker comes on and says something along the lines of “machine full, please discharge.” At this point, you’ll want to walk back up to the trap and hold X to discharge the energy and it’ll blast the door off.
Walk inside the room and hold X to pick up the weapon that the skeleton was holding, and you’ll get the achievement!
I know it’s a long guide, but I wanted to make sure it was easy to follow/understand. If you downvote, please say why so I can fix it! Happy hunting!
EDIT: As Maka91 and CelticCorpse has stated in the comments, only one person can acquire the weapon, but the achievement unlocks for everyone in the lobby. (This was also my case.)