The concept is pretty cool! Actually kind of makes me think who would win, haha.
The detail is pretty good and I like how you made the whole illustration purple and gold. What really got me, though, was the well done Namek background.
One thing I'd like to point out is that Mewtwo looks like it's not standing on the ground like Frieza. Their feet should be parallel with each other if they're both standing on the ground.