Stacking Durge

Does killing the undead enemies no longer work (i.e. Relentless Misfortune)? I am trying to stack up my Durge here, but it seems even when he empties their life bars they don't add to his stacks.

I also noticed these undead have a few health bars (sort of like armor) which I didn't expect. Maybe I am doing this wrong? Or does this method no longer work? I've been trying to farm stacks with him for days and he's only at 200k (i was using him with Shandie and HewMan in my active Gem Farm team just to get kill stacks..)

I'd like to stack him up so I can use him instead of Asharra in my pushing party (cuz honestly I'm tired of Asharra.) I'm a pretty new player so thought stacking him up might do me some good.... What gives? I tried to search and it seemed like everyone said use Relentless and let it stack up on a tank and let him feast.. It ain't working for me :( Maybe it was changed recently.