This is an enemy whose power is to adapt to any power used against it. Hit it with a fireball? Next time it's immune to fire. Same with any Elemental Rock-Paper-Scissors attack. Cut it, it now has armor. Psychic powers, it now has mindshields. Logic would dictate the "easy" way to beat it is to use an overwhelming attack to begin with. However, this goes against most heroic codes, so by the time they've slowly cranked up the power of their attacks, the villain has "adapted" quite a bit. Beings with an Adaptive Ability are very often Social Darwinists, if they're not mindless monsters.