Checks & Counters:
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Opposing Volcanions with Air Balloons is the most common defensive strategy to prevent Volcanion sweeps as it is immune to Volcanion's biggest assets: High SpA with Water and Ground coverage.

Rotom-Heat acts as an amazing offensive check to Volcanion as its base speed of 86 allows it to use Modest while still outspeeding max speed Timid Volcanion and OHKO'ing with Thunderbolt. It's ability, Levitate, introduces mind games once the Air Balloon on the ally Volcanion pops which helps in bringing it onto the field to apply potentially game-changing pressure. However, Rotom must be wary of Flame Charge Volcanion as another option to consider either in-battle or during teambuilding.

Mega Charizard X is the only Fire-type pokemon that is neither weak nor immune to Volcanion's STAB. Bulkier versions can be EVed to avoid the guaranteed OHKO from a crit Steam Eruption and get out of range from Earth Power after a 25% Stealth Rock switch in.