Dirks are the little green enemies (kind of looks like a bat?) that appear fairly early in the game. They take three slices to kill and only have a forward lunge attack. You can tell they are about to attack when they take a power stance for a brief instant. Lure one close to an edge then stand along it so you're facing the Dirk. Once it's about to attack simply move out of the way and watch the poor creature plummet to its inevitable demise.