What's giving you trouble?
: Run away, down tilt, up tilt (occasionally).
: Shield his aerials and run away. Don't get too predictable with mini fireballs or down tilt or you will get hit. Fair when you can.
: Pay attention to his zoning habits, especially with bombs. Try to catch them or reflect them. Throw Shadowballs. Fully charged ones beat his projectiles. Respect his disjoints and aerials. Mix up your landings against him.
: This isn't easy to be honest. Run away as best as you can and shield if you get in range of his attacks. Watch out for his bair and dash attack. Pay attention to the DK's grabbing habits. Make sure to have fully charged Shadowballs on deck.
: You should be able to punish her side b mixups after shielding it. The jump variation has a lot of recovery so punish it with Fair if you see it. Once again, PS if you get in their range. Don't spam down tilt as Fair will probably beat it.
: Shield dash attacks, punish him severely with down tilt into stuff, capitalize when off stage or when he is in the air.
: He doesn't have a lot of safe moves and his landing options are weak so practice punishing him when he throws out aerials. Don't get too greedy when attempting to juggle him as he can just yolo dair or yolo down-b. Try to stay out of his grab range at all times.
: Reflect her projectiles or throw fully charged Shadowballs, respect her fair and punish her aerials OOS as they are not that safe.
: Throw mini shadowballs to stop him from charging. Be wary of Cloud's spacing on Nair as it isn't always safe. Juggle him if you get a hit with down tilt.
: Whiff... punish...his...moves...or...bait...him. Don't go ham against him at all or challenge his range.
: Same as above, but it's a bit harder because he is harder to punish on shield and he is more mobile than Dorf.. Focus Attack is really gimmicky and loses to multi-hit moves (hint, Nair, Up-Smash).
All of these matchups require Mewtwo to run away and whiff punish hard, which he can do well against these characters.