Lorthemar and Varian have stupidly high amounts of HP. The only way they could be killed is if fracture isn't interrupted. Yes, they do taunt, pull aggro, and stand in fires, but it's all irrelevant as it amounts to only a few percent of their health pool.
All our NPCs usually end the fight over 80% health. We do not heal them or do anything special aside from interrupting Fracture.