Are you sure that you actually defeated him? Sometimes you do, but then the quest doesn't clear it for some reason and you have to do it over again. In some cases this happens because first you need to LOCATE the place that they are in, and if you teleport to a friend inside there, then that part will not be completed and you defeating the boss doesn't count. Other times you need to collect an item from a boss, and you don't find it when they are defeated so you have to re-battle them. Even other times you have to do something in the room they were in.
So to summarize that, go back and defeat him again, then explore his room and see what you can find.
I am having some problems with my quest as well --- Tree of life ---- talked with guard to cross bridge and meet inside --- does not update when i cross and have killed boss 2xs and still shows ruined bridge as not done --- what do i do? or what have i done wrong? Cant pass Tree of Life for some reason.