I had tenacity down on him and failed to apply the AoE daze or stun on him for about 10 minutes with my nightsisters and he kept counterattacking and regenerating protection non-stop. He was also bugged as a Level 1 still, but had insane health (screenshot below). Eventually I landed a daze with the special attack, but there is no way this is normal behavior (I've done this mission half a dozen times now). Just so people know, you might just want to use Darth Revan to deathmark or Nihilus to annihilate.
He is a Boss and therefore his stats are significantly increased. That is the intended behavior.