[Pokelink]

I don't know if it still works in the new versions, but I used shout (RD) to stun him and then I unleashed the most powerfull attacks, using also Pinkie's Party skill to maxize the options of Critical hits and reduce greatly his HP. If the stun doesn't work anymore... well, no idea.

EDIT: Nope, it seems like the stun isn't working as before. So, if you can't stun him, what else is left at this early stages in the game? He recovers quickly, have great defenses and heals about 500 per turn, and can cast confusion at will to the entire team, not only when you attack him directly and regardless of who attacked him. He can also use some thunder attack that can wipe the entire team in just two attacks