This is likely the easiest achievement in the DLC (in my opinion anyway).
All you have to do is get a werewolf (either wait till round 15 for one to spawn or open pack a pack a punch which guarantee's a werewolf spawn). Once you have your werewolf, take it over to the greenhouse laboratories (which is the area you upgrade the Alistar's Folly) and there will be a trap sort of opposite the main door. Make a werewolf walk over the trap, activate it and you'll unlock it if the werewolf instantly dies via the trap.
Video reference (by AC OMEGA
) : *NOTE* You can't unlock achievements when using modifiers, so when doing achievements do it on classic!