To play as the Werewolf, and thus unlocking this achievement, you'll need to kill a Ghost.
The only way to kill a Ghost is with a Ghost Gibber. The Scientist starts with a Ghost Gibber in his inventory (details on how to obtain the Scientist as a starting character can be found in my other solution
here), so be sure to use him to attempt this.
Much like the
Vampire achievement, you'll need to find a graveyard and start smashing the gravestones until a Ghost appears. Equip your Ghost Gibber and fire away until the achievement unlocks.