Did they change this quest somehow with mod 9?
Previously I always had to lure him to the light cage room on the other side of the map to be able to finish him off (and have a buddy or hireling to help with the arrangements).
When trying now, he came out from his coffin and I gave him a scorching ray to be sure I got his attention. Pof! Quest finished!
It was on normal, soloing with one hireling standing guard at the quest entrance.