When I first did this guy none of my mages had max defense and got raped pretty quick (Crushing blows almost 100% of the time combined with high crit rate), had to do the slow DPS and wait for the aggro change to ensure my guys didn't get into melee with him at all.

As it stands now I just spread my guys out and do full DPS the entire time. I use ice armor on the mages and just chain heal them while they finish him off (Tank beats on him the entire time from the rear)