[Coda]

I mean there's no specific "anti-Mewtwo Pokemon". But there are some things to take into account when designing your party, because just bringing in the recommended team isn't always wise.

Most importantly: Mewtwo hits EXTREMELY hard. Any Pokemon weak to its attacks is going to get one-shotted no matter how strong they are. Its Fast moves are all Psychic-type, but its Charge move can be Ice Beam, Flamethrower, Thunderbolt, or Focus Blast. You can get a hint at which one it is based on the recommended team (e.g. it was recommending Lapras for me because the Mewtwo I fought had Ice Beam). Having a type advantage defensively gives you more flexibility on choosing your team based on offensive capability.

On the offensive side: Mewtwo is weak to Dark, Ghost, and Bug. Ghost and Bug are annoyingly rare attack types, but there are a few options. Dark is far more common.

Tyranitar is the best choice available as long as it's not a Focus Blast Mewtwo, which will tear it to shreds in a hurry. Gengar is weak to Mewtwo's fast attacks so it's very fragile, but its powerful Ghost-type attacks can do a lot of damage before it gets taken down. Scizor is pretty good but it's ridiculously weak to Flamethrower. Pinsir is weak to Flamethrower but resists Focus Blast. Rayquaza is just massively powerful on its own and can just push through the lack of type advantage on raw stats, but it's very weak to Ice Beam. Houndoom and Sharpedo resist Fast moves, Ice Beam, and Flamethrower while having good offense. Absol and to a lesser extent Umbreon can do reasonable damage while resisting the Fast moves, but all four of these last choices are weak to Focus Blast, and Sharpedo has low HP and Defense. Banette can put out a good chunk of damage but it has low HP and Defense so even though it's not weak to any of Mewtwo's attacks it's still flimsy.

Finally, Mew and Mewtwo themselves are also good choices, as they're both very strong powerful, both resist Psychic and Fighting, and both learn non-Psychic offensive moves. Mew can get Infestation as a Fast move, which is a reasonably good choice.

So yeah, Mewtwo is a hard fight, yo. Three people with perfect teams isn't enough to do it. You need more people.