While trying to avoid ending quests, i ended up preparing a few bounties (from my warden's lodge).
I've been able to kill most of the monsters, but they didn't really die.
They lost all of their health, became untargetable, but kept throwing spells and pounding my heroes to death.
I tried several monster groups right after the savegame already delivered... in 90% i've been able to create at least one of those "ghost monsters".