Mewtwo doesn't like pressure or having to deal with disjoints. Watch out for Up Smash that thing will hit you if you get close, don't be afraid of Shadow Ball but do respect it and try to use any charging as a free approach or chance to charge your own things.
Edit: Mewtwo is also super lightweight so you will kill him slightly faster than he can kill you.
Things to be aware of are that Mewtwo will be able to punish FAir, and DAir, but not NAir and he can't beat UAir,
If you get caught with Confusion don't attack just jump and wait because usually a smash or a grab follows up Confusion. Be careful while crossing up Mewtwo, he can just OoS Up Smash or teleport away or Up Tilt.