I just waited two more levels and came back and finished it fast. Nothing says you HAVE to do this at lvl 19. It is hard to do it at that level. Easiest way is to have someone come help you and you go help them with theirs. You need to be able to stun her and take out the adds as fast as you can before she becomes unstunned. Or use some of the tips other's have posted here.
Missee