This mob takes forever to kill. It seems to buff itself with Aegis of Alsa Thel, which is next best to thing to a mob invulnerability spell. Cannot be dis-spelled.
I know this is late but the mob is not buffing itself. Part of this zone (and the quests around it) are the "thel ereth ril" running around buffing everything. Doing the progression gets you an item that dispells the buff and makes the "thel ereth ril" killable so they stop buffing.