Re: Boss Preferences
SquareSoft did something like that in Final Fantasy IV, where Light Cecil had to not attack Dark Cecil in order to win. All the player could do was heal himself to avoid dying. If you attacked Dark Cecil, it had no effect.
That being said, if you're going to pull a "twist" like that, you generally want to give at least some foreshadowing of the event so that the player isn't frustrated that this particular creature doesn't follow the rules of the world.