TL;DR: You need a lot of coverage, with Baton Passers to back Mewtwo up.
Mewtwo @ Mewtwonite X
Ability: Pressure
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature / Jolly Nature
- Thunder Punch
- Ice Punch
- Psycho Cut
- Drain Punch
OR
Mewtwo @ Mewtwonite Y
Ability: Pressure
EVs: 4 Def / 252 SpA / 252 Spe
Modest Nature / Timid Nature
- Thunder
- Blizzard
- Psystrike
- Grass Knot
Mewtwo X only has one Pokemon who is immune to all his attacks, Shedinja. None are hit not very effective by all, 235 are hit with normal effectiveness by all attacks, and 564 that are hit super effectively by at least one, some being the all powerful Dragon types and things like Mega Garchomp and Tyranitar.
Mewtwo Y also has one Pokemon immune to its attacks, but 3 not very effective, the Magnemite line, 344 neutral, and 452 super effective.
An Assault Vest could also work, but I recommend mega stones.
I have also found this set works well with two Baton Passers before it, Drifblim and Scolipede. Drifblim has this set:
Drifblim @ Bright Powder / Sitrus Berry / Mental Herb
Ability: Unburden
Timid / Jolly Nature
EVs: 252 HP / 252 Spe / 4 SpD
- Minimize
- Baton Pass
- Substitute / Destiny Bond
- Stockpile
This helps because the EVs for Mewtwo go into Attack/Special Attack and Speed, not Defense.
Scolipede @ Leftovers
Ability: Speed Boost
Jolly / Timid Nature
EVs: 252 Spe / 252 HP / 4 Def
- Baton Pass
- Substitute / Protect
- Swords Dance
- Agility / Iron Defense
This will help Mewtwo sweep.
If you use Mewtwo Y, Scolipede will not work.
You need Ambipom.
Ambipom @ Leftovers
Ability: Technician
Jolly / Timid Nature
EVs: 252 Spe / 128 SpD / 128 Def
- Nasty Plot
- Agility
- Hone Claws for accuracy or Substitute for protection.
- Baton Pass
The Hone Claws can be very helpful because Blizzard and Thunder have low accuracy.
You could also go with Fire Punch on Mewtwo X for Shedinja, but I don't recommend it.
Focus Blast on Mewtwo Y instead of Grass Knot could work, but it depends.
After all this, Mewtwo only has a 1/3 chance of being hit by attacks, and can wall them out while still sweeping like a pro.
Hope I helped :3