Yep Marshtomp counters it hard. Only Electrode and Emolga are above the lvl cap. I used the same pokemon/strategy like I would normaly. 2xTrubish for Toxic Spikes/sludge, Kricketune, Marshtomp, Goldeen, Espurr, Litleo...all were lvl 19/20. I managed to beat her with Monferno too.
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That is all you need to know.