If you find yourself battling someone using 6 Mewtwos, you shouldn't even consider wasting your time on such a pathetic loser.

If you're pondering over defeating a single Mewtwo, then I'd suggest starting off with an Electrode, to guarentee a Paralysis on the first turn. The switching to a heavy hitter like Snorlax or Tauros, or using Chansey's huge special and HP to absorb special attacks, heal with Soft Boiled and run down the enemy with SToss or special atacks.

PS. Snorlax can only learn Self Destruct, not Explosion.