A Magnemite with Thunder Wave + its Sturdy ability can help you with that fight, if the Mewtwo is paralyzed you can outspeed it easier and as that Mewtwo is Psychic and Steel type its weak to Fire, Ground, Ghost and Dark type attacks, try it with some Fire type moves from your Charizard.
You could also go the "lucky" route and hope that it stays paralyzed and use the move Curse with a Gastly, you might need some potions for that too though.
Regarding training I would recommend the Trainer Tower, you can find it on Route 13 for Kanto.
Good luck~