Pinnacle of its kind - Primary demon fight does not trigger

I had the same problem, only I died so it teleported me out of the room to the last waypoint. It said I have to defeat the dragon, but she was not in the arena.

I went away, did some quests since checking it last time, then she was there. I suggest you to check back sometimes and see if she is there. I was not expecting the bug to go away, but it worked for me.