In the sixth gym city the poke center has a path leading from it to a lake then you use surf and go south until you can't go anymore. Then you go a west direction until you get to a cave. Saffron islands is that cave. You have to keep pushing boulders down to stop the current. Once you've done all that you use to get articuno once there use your master ball . And don't worry about mewtwo I have a plan for that too. ENJOY!