** 14/03 SPOILER **
After a second thought, I tink that there would have been a way to kill durr mixing corrosion and burning (which I didn't think of in game). I am not very good with these effects (is it possible to fireball him ?)
Edit : no, you can't fireball magic immune monster. So it would have required stacking enough corrosion on him, an then placed a monster at the right place with wonafit, to one shot him once you would have killed hurr.
Edit : Hmm. Corrosion adds to damage when sending something at a monster, but does not help removing the last health point. So it wouldn't work either
** End of 14/03 SPOILER **