Need advice on killing Fildgor Orcthane

Kingcroft

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.

Kingcroft

For some reason I was able to beat him in a rather strait forward way this afternoon.

Parmie

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.