Instructions
Compete in the easiest difficulty level of the Subspace Emissary mode.
Complete all levels of the mode, which will unlock all of the secret characters and the "All-Star" mode.
Compete in the "All-Star" mode on intense difficulty. In All-Star mode, you will randomly fight every character in the game. After each fight, use one of three hearts in the hub room to restore your life. Only use one of the hearts if you feel that you will die in the next battle.
Complete the "All-Star" mode and the Mewtwo trophy will be unlocked.