O.K. the NPC's with 100% resist everything you can lure away from the stairs, run down - not great, because in doing that you lose the XP you desparately need to level but ... that's a choice you can make.
Eternal bone giant with 100% resist everything regen and fearscape ?. Please

Easiest fix, cap the resistance on the NPC's to 90 maybe 95%, if the player resistances are capped, NPC's should be as well.
It was challenging up to that point, which I didn't mind, but that, excessive

Pete