I understand folks have said that this particular quest is bugged.
I too am one of those.
When I started the quest to stomp Sinmur only one of the NPC's was working. The dps caster it's name fails me. Seeing Sinmurs health and only being lvl 37 at the time I was fearful that I would die. My first attempt I was being crushed and he would 2 shot me. So I paused for a moment and thought that here have got to be other quests that I have not completed to get these other NPC's working. Sure enough I started to find them. Each of the quest's I had started and completed had what I'll call distictive "red/burgundy tents" low and behold each of the NPC's that I helped/completed quests for in turn helped me to finish off Sinmur. This is not to say that the final quest is not bugged or shall we say working as intended. Having to kite the Sinmur around for 30 minutes was not a fun experience. The NPC's would hold aggro for a short period of time, and then Sinmur would come running after me. So my strategy as was to knock at his ankles while the NPC's had his attention avoid bad and when his attention turned to me I ran while kiting him I would cast and slowy but surely he died, this was a rinse and repeat process and I was kiting him everywhere I could to keep him at distance. I was able to continue on my journey, and now questing in Coldharbour.
I hope this of some help to folks who are having some difficulty with this one.