I would suggest a strong DARK type pokemon, because then Mewtwo wont be able to use its Psychic attacks.
Master Ball
any strong Pokemon
You can't catch mewtwo in Pokemon diamond. It's better if you use Action replay code.
Mewtwo is at Level 70 when you battle him. You should most likely use you highest level Pokemon that is around level 70.
It isn't meant for any one pokemon. Most people use it on Mewtwo, which you probably should use it on. You can catch the legendary birds and Mewtwo without a Masterball but it can take a long time.
Use Action Replay
masterball
First you got to catch him in Pokemon leafgreen or fire red and trade. Or you can use a gameshark. it lets you catch all the Pokemon from bulbasaur to deoxys.
Well, if I where you I would use it on Mewtwo in Cerulean cave after beating the Elite Four when the guard is not there, another good call is to save it and if you get any event Pokemon use it there
you have to save your masterball no matter what and use it on mewtwo in order to catch it faster. otherwise, you are going to spend hours trying to catch it without any luck.
ANSWER 1:Mewtwo appears only in Fire Red and Leaf Green. You can catch him in either of those games and then trade him to Pokemon Ruby.sorry mate its impossible only way is to trade like this guy said (: (: (: (: (: (: (:ANSWER 2:I regret to tell you that you cannot catch Mewtwo in Ruby. You can only catch Mewtwo in FireRed or LeafGreen. (You can use cheats to get it but it wouldn't be as fun).Mewtwo is at Level 70 in LeafGreen and FireRed so use an Ultra Ball/Timer Ball/Master Ball.If you need/want a Mewtwo you can trade Mewtwo into your Ruby/Sapphire/Emerald Game.
You can't. the only way is by the use of an event which has long passed.