The healing-decay mechanic is broken.
Unless they patched it recently, the experience I had with the witch in the cave, decay is applied when your physical armor is broken. Heals bypass both magic armor and resistances. You shouldn't be able to cheese mechanics to instantly blow up a boss (I shamelessly did it on that fight). At the same time, player characters have a lot less health, armor, and resources, and death is expensive. Damage shouldn't be that devastating with the limited resources at hand.