Hardest was Kabu, even though I had Mareanie who just laughed at Centiskorch's attacks, at the time I had Skorupi and Roselia who couldn't help much against him, and sadly Mareanie didn't know any Water type attacks. So it was a long battle and slightly annoying battle.
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Easiest definitely Opal no question about it and I honestly feel bad, because I had literally just gone through a forest full of Fairy types, where I defeated anything that came across my path and everyone was reaching the mid to high 30s evolving one after another. I had just defeated Alister with Croagunk and on the way through the forest he evolved into Toxicroak.
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I can write up my playthrough a little.