I haven't found a way yet to interrupt his healing. So far all CCs I've tried (Knock back, Stun, and Drag) he resists every time. Am I supposed to use an elemental cc like lightning or frost or do I just have to respec and get cc penetration? Also, I tried to attacking to see if there is a certain dmg threshhold but he still just kept using his healing ability and was not interrupted. Thanks all!
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use rune "poison" to make your weapon damage poison element, i use a giant shield with poison rune, i notice that the damage gets higher while im hitting the boss with melee skill ( giant shield ). start from 600 dmg, it goes up to 10k poison dmg.