Instructions
Complete the Pika Cup in Stadium Mode. You can only use Pokemon between level 15 and level 20. In addition, the combined level of your team cannot exceed 50.
Complete the Petit Cup in Stadium Mode. You can only choose Pokemon between level 25 and level 30. The cup also requires Pokemon participants to be no taller than 6 feet 8 inches, and no heavier than 44 pounds.
Complete the Poke Cup in Stadium Mode. The official Pokemon League tournament allows you to choose Pokemon between level 50 and level 55. To complete the cup, you must win in four subdivisions: Poke Ball Division, Great Ball Division, Ultra Ball Division and Master Ball Division.
Complete the Prime Cup in Stadium Mode. This cup has no level restrictions. Once again, you must complete four subdivisions.
Complete Gym Leader Castle. First, you must defeat the eight Kanto Gym Leaders: Brock, Misty, Surge, Erika, Koga, Sarbina, Blaine and Giovanni. Next, you battle the Elite Four: Lorelei, Bruno, Agatha and Lance. The final challenge of Gym Leader Castle is a battle with your rival.
Return to the map screen. Mewtwo's tournament should now be accessible.