Zarnas Stonewrath can prove to be a difficult boss to defeat. He casts Aura of Restoration and constantly uses Stone Daggers which proves to be very effective at healing him. Removing the Aura of Restoration enchantment will make him much more vulnerable, as will disabling his use of Stone Daggers.
Henchmen have a serious pathing problem in the area where Zarnas Stonewrath resides, a narrow corridor filled with large columns, they will tend to remain stuck behind those columns rather than going around them, if this happens they will simply do nothing at all. Simply go back and move around a bit.