The quickest and easiest way to killing the Witch King is to lead him into the fire. When he is directly on it, hit him once. Do not worry if his armor resisted the blow. The Kitch King will use a power surge and the fire will drain his life. Repeat this until his life meter is almost gone, then begin using your sword to attack him. After you defeat him, go to the man who was killed at the beginning of the level. Search him to get the Witch King's Crown, which is an Artifact.