Not sure what was going on. My 31 Mage attacked this one with a fire pet. Something kept healing it.. No "so and so begins to cast a spell" messages seemed to correlate with the heals. Finally took it down, with 20% mana left and my pet at 10% life.
Youve prolly figured it out by now but a nearby page, justicar or judicator was within range of healing its friend but not aggroing your pet. Definately a pain the ****