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I get him down to 1/4 health, then the dark spirits start to heal him, I take them out but he still is invincible not taking any damage from any of my attacks, ultimate or other wise, please advise.
I had the same issue, some intuition told me it was my vampiric drain attack either on him or the ghost adds, when I didn't use it at all in the fight, I did it without huge difficulty although by then I was seriously over level for it.