Bisharp is your man.
Mew2slayer (Bisharp)@ Focus Sash
Adamant | Defiant
4 HP / 252 Atk / 252 Spe
- Sucker Punch
- Knock Off
- Iron Head
- Low Kick
Your friend's Mewtwo has all attacking moves. Therefore, a Sucker Punch is gurarunteed unless he switches and guaruntees a KO. Knock Off actually has a chance to KO too if you fear a switch.
252+ Atk Bisharp Knock Off vs. 4 HP / 0 Def Mega Mewtwo Y: 158-188 (86.8 - 103.2%) -- 25% chance to OHKO
If you're afraid he'll predict Sucker Punch and switch Mewtwo out, you could always trap it with a move like Block, Spider Web or Mean Look. Meowstic-M gets both Prankster and Mean Look, ensuring that you trap Mewtwo.
>But when Meowstic switches to Bisharp, the trapping effect wears off.
^ I forgot this. If you're desparate, Bisharp gets Mean Look, and the Focus Sash ensures you get one in.