I got this one on level 6 (Hessian Castle: An Ancestral Home).
Play through the level until you get to the locked door. There will be two guards patrolling the area with more on upper levels. Dangle from the post (be careful of the sudden flashes of lightening) and wait for one of the guards to walk passed and kill him. Once the second guard sees him and gets terrified, restart from the checkpoint and do it 4 more times.