User possesses extreme regeneration, allowing them to not only recover from any wound almost instantly, but change one's form which can allow them to gain new powers or become stronger.
After suffering brutal experimentation, Killer Croc (Batman: Arkham/DC Comics) mutated even further upon regenerating.
Gerard Valkyrie (Bleach) can use "The Miracle" to continually increase his power by converting the damage he takes into strength. Even being bisected vertically and being ripped to pieces only allowed him to reach even greater levels.
After being mortally wounded by gunfire, William Birkin (Resident Evil 2) infected himself with the G-virus, not only healing his wounds, but also mutating into a creature of incredible strength...
...As William became increasingly injured during his fights with Leon S. Kennedy and Claire Redfield, the G-virus rapidly healed and evolved his body to that a clawed juggernaut...
...In his third form, the G-virus mutated William in a hulking, multi-armed brute with his humanity all but lost...
...At this point in his fourth form, William had already sustained a lot of damage, forcing the G-virus to compensate mutate him into a ferocious monster of claws and multiples eyes...
In his fifth and final form, William has sustained significant damage to the point where the G-virus was unable to fully heal him, leaving him to become nothing more than a mindless mass of flesh and teeth.