First, meet up with him in his room. Then, lead him to one of the walls that look like cage walls. Go on the other side of the wall and close the door behind you. Shoot him with a bow or use magic. Since he cannot open doors, he will just stand there. Just keep shooting him until he is defeated.
Find the Butcher. Open his door and run away from him. He should follow you. Lead him to a staircase and run around it, but stay close to it. He should follow you in circles around the it. Eventually, after about thirty seconds to a minute, he should get stuck in the staircase. From there, shoot him with magic or a bow. You should also be able to melee attack him. Note: He can get away from it if you go onto the side of the staircase that you need to be on if you were to enter it.
Open the door, cast Fire Wall immediately, then close door after the spell. The Butcher will be stupid and stand in the flames provided you stay at the doorsteps. He will die almost always before the flame goes out, even if spell is cast by a Warrior. Note: You must be fast and need to proceed a few levels down in the dungeon to learn Fire Wall.