I would try to mutate to a beast equivalent to William Birkin
What if you were Peter Griffin?
Mewtwo stomps since his stats are higher than Deoxys'.
Mewtwo's stats - 680
Deoxys' stats - 600
Also in speed form, Deoxys is only 30 points higher than Mewtwo...so Mewtwo is not getting blitz.
All Mewtwo has to do is one or two Shadow Balls and he wins.
Also, Mewtwo has his spoon that boosts his abilities as well.
@The_Legendary_SuperSaiyan_Hulk: True, well Jellal has kill spells and IMO is one of the most powerful wizard in the manga.
We also have the Celestial Spirit King who can manipulate time and possibly space... he is also a creator. Now he is the most powerful character in FT IMO.