Easiest way for a Mage: Freeze and one Hell Spike
Easiest way for a Warrior: Lead them to somewhere that you can corner them and hack away (You must be able to do considerable damage to them or they will just band their drum and regenerate their health). A couple of points in Freeze will help a Warrior a lot (I think they can gain the Freeze skill - I never really tried it when I played as a Warrior).
