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
Yes. It is completely possible to catch Mewtwo with an ultra ball.