Push a zombie over a railing.


This can be done in a few places, but the one that makes most sense is on Act 2 Scene 8 (Lydia). You will obtain a fire axe on this scene. Once you do, break the lock and kill the first enemy you see. Continue past the other ignoring it. Break the other door down and get the enemy over near the railing. Get on the left side of it and push it until it falls over the railing.