I just finished that one. On Paladin, actually. Wasn't that hard.

Stay on the outside edge and just keep running back and forth. As soon as his arm starts to slap down, roll. If he has a rock, or if his health bar is gray and you're about to go into a QTE, roll and then immediately jump so you don't get hit by the shockwave.
Took me a few tries, but I managed it in about ten minutes. If you're really having trouble, save a dark crystal for the second phase of the fight, then you only have to dodge the first "section" and his fist slapping down the second time.