Put a Dark-type Pokémon with False Swipe (I recommend Absol) at the front of your party. Use False Swipe until Mewtwo is at 1 HP. Start throwing Dusk Balls.
MasterBall
mewtwo
The best way is to get some codes online. Using an online forum is a great way to do this.
The best way would be to catch Mewtwo with a MasterBall. However, you can also catch it with Ultra Balls, but that will cost you a lot of them. Mewtwo is a psychic pokémon, so it's weak (2x) against bug, dark and ghost types. It's highly recommended to breed a Rattata so it learns the move Endeavor. After Mewtwo has 1 hp left, due to this attack you can put it to sleep, freeze it or paralyze it.
With a master ball.
You can't catch Mewtwo in Pokemon Diamond and pearl and also platinum. But you can catch Mewtwo in heartgold and soul silver
how do you catch a mewtwo on Pokemon platinum
The only sure way to instantly catch any Pokemon is with a Master Ball.
you can't catch mewtwo he is too tough to catch but if you don't listen to me you are a NOOB
i have a mewtwo here and it isn't really hard to catch.... rayquaza is really hard to get but you can see it on your way... pikachu, i guess, is the hardest because we can't catch it by only one try...
MewTwo is really strong. Try to get your hands on a master ball, as that's the only way people i've known have done it
In Pokémon FireRed, the best Poké Ball to use for catching Mewtwo is the Ultra Ball, as it offers a higher catch rate than standard Poké Balls. If you have access to a Master Ball, it is the guaranteed way to catch Mewtwo without fail. Additionally, using status effects like sleep or paralysis can increase your chances of a successful capture. Be sure to lower Mewtwo’s HP carefully before throwing your Ultra Balls.