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Diantha's way of fighting canonically changes depending on the Pokemon she uses. for Round 1, she was defensive and it made sense knowing how Lance never runs Brick Break on his team. Leon's Charizard has and can relearn the move, so Reflect and Light Screen are absolutely useless. Hence, she will obviously go for offensive play this time around. most likely Pokemon to be used based on merch (first 2) and foreshadowing (latter 2):
>Hawlucha (jobber)
>Tyrantrum
>Gardevoir
>Gourgeist
assuming 2 gimmicks for semis, we can have Mega and Z-move kino here. Gourgeist may be the last one standing, of course ultimately losing to CHARIZARD, but not without a good showing of her signature Z-move Trick or treat.