how about a creature that regenerates everytime it successfully hits you?

There is a construct that heals itself periodically up near Muspar'i..

An ashu hhinvi crouches its lumbering hulk close to the ground, placing its paw-like hands on the sand. Waves of sparkling particles funnel upward into the creature's colossal form to fill out the cracks suffered in the heat of combat. Replenished and healed of its most injurious wound, the ashu hhinvi stands erect once again, more healthy and agitated than before.