You have to win on the third Shadow Mewtwo fight. Afterwards, it locks out the SP league for a while while you do the remaining story fights.
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3000 HP Shmewtwo. Thanks Bamco. At least you had the decency to weaken his attack and AI for it.
R1 No Synergy burst. Must be won.
R2 AI goes aggressive. Impossible to win this round, Shmewtwo becomes invulnerable to your attacks if you get him to the halfway mark.
R3 Unlimited Synergy Burst for both of you. Must be won.
Used Sylveon assist for this one. The added survibality becomes welcomed here.
Holy crap, this is a lot of work to unlock normal Mewtwo and there's still the last league left.