Decapitator “One Shot” should just be internally reworked to:
If an enemy dies due to an instance of damage delivered by this weapon that exceeds its hit point maximum, give the wielder a stack.
You kill a 600/600 health enemy with a 615 damage attack → +1 Stack
You kill a 200/600 health enemy with a 615 damage attack → +1 Stack
This way you do not hate your teammates in this co-op game when they do aoe damage to enemies, removing your ability to gain stacks.