when i was playing actively, i practiced the mewtwo/corrin matchup pretty heavily. here's what i remember helping:
-make heavy use of grabs to condition them not to counter so much. punish each counter with a forward throw or up throw if at kill percent. focus entirely on positioning and go mostly for single hits until they stop countering so much, at which point you can start mixing in a few combos (not too many, you don't want to get predictable)
-fight a cpu corrin and run away as much as possible so they will fire dragon fang shots at you, and get a feel for reflecting them. once corrin is stunned, attempt to punish with a jump cancel up smash, and get a feel for what ranges you can do it at
-charge and fire shadow balls as much as possible to prevent them from getting into pin range
i don't remember everything and i'm sure the meta has changed but by following this i had a lot of success in the matchup