1.beat pokemun league b sure you got 60 pkmon
2.get the ruby and saphire to cielo to clear the cerulean cave.
3.go to cerulean cave and find mewtwo (you will see him standing)
4.catch him (master ball, ulta ball)
5.train him until it reaches level 90
6.the best move for him is flamethrower,icebeam,thunderbolt and psychic.

BECAUSE:

thunderbolt beats the ice trainer use also flamethrower to the ice type pokemon like jinx
flamethrower again for the steel trainer and psychic for the fithing pokemon
use thunderbolt to gyarados and use icebeam to dragonite or aerodactyl.
then choose specific move to gary because gary trains differnt kinds of pokemon.

pls email me at sacredguy_69@yahoo.com for some guides.