9Tales
Smash Apprentice
There are actually a lot of things Mewtwo can do.
1) Make your return trajectory unpredictable. Like with most floaty characters it helps to mix up fast falling and regular falling and to switch to one from the other mid fall. It throws off your expected trajectory and messes up attempts to hit you with aerials. With Mewtwo you can additionally throw in a random mid air confusion to further mix up your trajectory.
2) Teleport. Yeah this move is pretty sweet, practice Teleporting down and diagonally down to the left or right. Mewtwo has relatively little lag and a lot of sideways momentum if he comes out of teleport on the ground. You can teleport form the air onto the ground and this can throw off the opponent in a similar way. Again mix this up though, if you teleport down every time it becomes punishable.
3).. so 1 and 2 are extra effective with opponents that try to juggle you with aerials, but if your opponent is the kind the waits on the stage and mirrors your horizontal location then you can always return to the ledge and try your luck with a game of ledge, paper, sissors. Alternatively opponents who play this game attack based on timing you know for sure, the time you land (as opposed to jumping up at any point during your fall) and so depending on the character and opponent you can try to beat out their attacks with yours as you land. (Confusion also still throws off opponents trying to do this to you pretty well, it's basically a mini 3rd jump)
4) watch how your opponent juggles you and expect them to recycle tactics that worked for them before. If they baited out an air dodge form you and you got punished for it then try delaying (or not even using) your air dodge the next time. Some times this causes them to never even go in for a hit. And on that note when you do air dodge through and opponent try to position your self so that you can land a well spaced bair as you come out of the dodge.
No offense was intended in this message.