In an unusual reversal of roles I got to ask another blogger to write a blog entry answering one of my questions. Shalkis mentioned in a comment here that he kills Shartuul whenever he has the necessary Darkrunes. I knew that encounter, had tried it a couple of times, but failed every time in the second or third stage. So I asked Shalkis to write up instructions on how to kill Shartuul, which he did. Thanks a lot, Shalkis, much appreciated!