Quintus Verres is the guy that summons the dragon. Have killed him as the dragon comes down and his body is there dead but yet you dont get credit for him dying.
I have not had the issue with the lion boss. Hes always been there.
Supposedly, what happens is you kill the guy, but the second boss gets stuck up on top and never drops down. People say that if you go back as a group to the wall area the final boss will drop down and you can finish the dungeon.