@Drakes Here are their weaknesses:
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In Rematch: Froslass and Drifblim
All of Shauntal's Pokemon are weak to Ghost and Dark. Froslass, Chandelure and Drifblim also share a weakness to Rock.
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In Rematch: Sharpedo and Drapion
All of Grimsley's Pokemon (other than Drapion) are weak to Fighting, use Ground attacks against Drapion. Bisharp is 4x weak to Fighting.
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In Rematch: Bronzong and Metagross
All of Caitlin's Pokemon (other than Bronzong and Metagross) are weak to Dark and Ghost. Gothitelle, Reuniclus and Musharna are also weak to Bug. Use Fire attacks against Metagross and Bronzong (Bronzong has Levitate).
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In Rematch: Breloom and Toxicroak
All of Marshal's Pokemon are weak to Flying and Psychic. Breloom has a 4x Flying weakness while Toxicroak has a 4x Psychic weakness.
And for Alder:
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Keep in mind that half his team (Volcarona, Accelgor and Vanilluxe) are weak to Rock. Use Fighting attacks on Bouffalant, Ice on Druddigon and Fire on Escavalier. Volcarona will probably fall easily to Physical-based Rock attacks due to low Defense and a 4x Rock weakness, avoid letting it set up Quiver Dance.