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Think you forgot to put Specs on Volcanion. Wouldn't really call Gyara a counter though as Steam Eruption has a stupid burn rate and Sludge Bomb still 2HKOes.
252+ SpA Choice Specs Volcanion Sludge Bomb vs. 60 HP / 0 SpD Gyarados: 162-191 (46.8 - 55.2%) -- 14.1% chance to 2HKO after Leftovers recovery
plus, it looks cool