Mewtwo is not a Pokemon which can be found in Pokemon Diamond through normal gameplay (encountering it in the wild). It is one of the select few Pokemon which must be migrated to Diamond through the Pal Park from a Kanto region game.
Migrating it from FireRed or LeafGreen (or Ruby, Sapphire or Emerald if it has been traded to it) to Diamond is the only way you can obtain Mewtwo through non-cheating methods.
Action replay.
you can't, you can only get mewtwo off action replay sorry
You can't catch mewtwo in Pokemon diamond. It's better if you use Action replay code.
There is no such pokemon as Mew-Three. But there is Mew and Mewtwo.
What you have to do is find the code type it in the it will start you.But you have to find it!!!
12ff43h65g1a31c what the hell is that suppsed to be that's not a code.
To Catch Mewtwo You Need A Action Replay.
if you have the action replay
Its impossible to actually catch one in diamond But you can transfer it from fir red or leaf green or you can go to the GTS in jubilife city if you have seen mewtwo in your pokedex
Action replay
Action replay is not an Pokémon item that can be obtained in Diamond.
Use Action Replay