Use an aerial attack in a way that causes you to land on a platform which would not have happened without the aerial attack. The reason is a downward shift of your character's environment collision box (ECB) bound to the attack or, more specifically, your character's bone alignment on a given frame.
Mewtwo is a poorly designed character, having almost no kill moves and no neutral game. Mewtwo only has a few redeeming qualities, such as his amazing recovery and powerful grabs. It is very hard to combo with/against Mewtwo because of his lack of speed and floatiness.
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