in the end of the cureiliun cave

u have 2 beat elite for with 60 or more pokemon and train then go back and face the elite
4 they r tough medium 60s 2 low and medium 70s

1.beat elite 4
2.give celio the sapphire and the ruby stones
3.now go to the curlean cave and and find him
4.his at the end
5.mewtwo is in lvl.70
6.save your masterball from your journey
7.use it on mewtwo and there you go