An absolute monster! No Pokemon is a better Special Sweeper, except for maybe Kyogre. Possessing a base 154 Special Attack (higher than Kyogres!), a lightning fast base 130 Speed, great HP, passable Bulk and even a usable attack stat, Mewtwo is a perfectly balanced god. Don't think it's a Special Sweeper only though, the good bulk allows it to pull of a more than brilliant Support set. Mewtwo, it may no longer have the same typing and base stat total as Arceus or Kyurem, but it is still one of the greatest pokemon ever!

252 HP, 200 Speed, 56 S.Defense
Timid Nature
-Reflect/Light Screen/Psystrike/Thunder Wave/Fire Blast

Taunt prevents hazards and status. Recover heals yourself. Toxic or Will-O-Wisp... Will-O-Wisp goes well with Thunder Wave for a Pseudo-Double Powder. See, if you Will-O-Wisp like Zekrom or something, it will most likely switch since it's useless. Thunder Waving right after the Wisp will paralyze the switch in hopefully. Even if they dont switch, they probably wont damage you too much, so don't worry too much. Toxic ridicules walls not named Ferrothorn.
Reflect boosts your defense if your not running Will-O-Wisp, even if you are, it's not as good as...
Light Screen. Protection from the omnipresent special attacks. Great!
Psystrike is your strongest STAB and kills special walls. Radical.
Thunder Wave for Pseudo-Double Powder
Fire Blast kills Ferrothorn.