[WildAce]

Originally Posted by Near

No customers?
Then Mewtwo.

Mewtwo@Choice Specs
Modest
252 Sp.Attack 252 Speed 4HP

Aura Sphere
Ice Beam
Thunderbolt
Shadow Ball

That leaves her with only Parukia that she cannot hit [and potentially Aruseus]

I'd opt for Timid to outspeed Darkrai and junk to the kill.
Beyond that, a ballzy version could be to drop TBolt for Charge Beam and replace Specs with Orb / Lefties.

Mewtwo@Life Orb
Lonely/Hasty
160 Attack 92 Sp.Attack 252 Speed

Focus Punch
Psychic
Shadow Ball
Substitute

Something like that.
I think Hasty is preferred for Darkrai outspeeding.

Mewtwo@Leftovers
Modest
252 Sp.Attack 252 Speed 4HP

Taunt
Calm Mind
Ice Beam
Pulse Bomb

Bleh..

Here's what I got:

Mewtwo @ Life Orb
Ability: Pressure
Nature: Timid
Evs: 252 Sp.ATK / 252 Speed / 4 HP
- Ice Beam
- Thunder / Thunderbolt / Charge Beam
- Shadow Ball
- Aura Sphere

Basically, this is an Opener Mewtwo. 394 Speed allows it to outspeed even the likes of Darkrai and OHKO it with Aura Sphere. Thunder should only be used when you have Kyogre on your team (or if you feel Kyogre will be on the opponent's team). Beyond that, TBolt is superior for instant damage, but the 63% chance of getting a Sp.ATK boost from Charge Beam is VERY inviting.

Mewtwo @ Life Orb / Leftovers
Ability: Pressure
Nature: Timid
Evs: 204 HP / 252 Speed / 52 Sp.ATK
- Calm Mind
- Substitute / Taunt
- Shadow Ball
- Aura Sphere

This is basically the Anti-Blissey Mewtwo. Sets up 101 Subs, Calm Minds up to heaven, then Sweeps away. It's quite effective, but not that easy to pull off. If you fear Roar (from other Pokes) more than Seismic Toss, then Replace Substitute with Taunt.