I don't know if it's a bug or not, but I could successfully do Gluth 25 while kiting him around his room. If you have Death's Advance and run around strafing you can DPS Gluth while you kite the adds. Another helpfull thing on that fight is Glyph of Death and Decay, making Death and Decay Slow all the adds, you could also instead of Death's Advance go for the slow and spread diseases with Roiling Blood + Pestilence Glyph.
So, be blood, spread either slow diseases on the adds, couple it with DnD if needed or go Death's Advance. I killed him twice now with just that tactic.
Or you could just use all Death Runes on Necrotic Strike and be done with it in no time. It's the only fight that requires pre-planning.