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???
They trained using the failed dance rhythm.I mean to be fair back in X&Y after his he lost to Korrina and her Lucario before they gain the ability to Mega Evolve he didn’t really train for a rematch instead we got episodes focus on Korrina mastering Mega Evolution.
Sure his Fletchling evolved into Fletchinder and he got a Hawlucha before the Gym Battle and yet he didn’t really do that much to prepare his Pikachu and yet it somehow wound up winning against Mega Lucario with no onscreen training for the battle.