As a mythic Pokémon, Volcanion is never seeing the light of day in VGC without another just for fun format which would include the numerous restricted Pokémon that bury Volcanion in stats. Which is a shame, as its melding of potent offense and bulk make Volcanion a potent choice in formats not rife with Miraidon and its ilk. Of most note is Volcanion's ability to spread burns, with access to its signature Steam Eruption, making the Hydro Pump strength move even more difficult to switch into as Grass types like Ogerpon and Rillaboom want nothing to do with the burn and even Grass type's mildly inconvenienced by said burns like Sinistcha, Amoonguss, and Whimsicott run in fear of Volcanion's Fire STAB. 80/120/90 bulk isn't bad, allowing Volcanion to fire off its powerful attacks repeatedly, making that burn all the more likely. Water Absorb is also handy to possess, making Volcanion aggravating against Water attackers while boasting unique synergies with Terastalization. Volcanion's Speed is an awkward sticking point, being way too fast for Trick Room, but slower than most other Pokémon, leaving it commonly outsped and forced to tank attacks. While Water Fire STABs are aggravating to the most prominent Water resistance in Grass, opposing Water and Dragon types fair much better especially given Volcanion lacks direct coverage to deal with them.
Burn Engine
Volcanion
-Steam Eruption
-Heat Wave
-Earth Power
-Protect
Ability: Water Absorb
Item Attached: Life Orb
132 HP / 252 SAtk / 124 Spe
Modest Nature
Tera Type: Ground
Volcanion makes for a great bulky attacker, blasting down opponents with its potent STABs while shrugging off lighter hits. It cannot be stressed how hard it is to switch into Steam Eruption without an explicit immunity. Hydro Pump power with the ability to Burn dealing horrific damage, doing even a 1/3 to targets as bulky as Raging Bolt and Assault Vest Archaludon (who resist mind you) as well as Assault Vest Iron Hands. Heat Wave applies appreciable spread damage while incinerating any Grass type that thinks they can endure Volcanion's assault. Earth Power provides coverage against the numerous Electrics looking to strike down Volcanion quickly, especially Raging Bolt who is a difficult matchup otherwise. Protect is Protect.
Ground Tera is recommended here, pairing wonderfully with Water Absorb to turn Volcanion into a pseudo Water Ground type while enabling Volcanion to obliterate Raging Bolt and Iron Hands safely with a super charged Earth Power. 124 Speed is necessary to outspeed uninvested base 85s like Rillaboom and Archaludon while outrunning neutral turbo Ursaluna and neutral Dragapult in Tailwind. Max Special Attack to emphasize Volcanion's best selling point and everything else into HP for bulk.
Other Options:
Sludge Bomb is great for Ogerpon-Wellspring, the other Water Absorb user running around, forcing it to Tera (opening itself to a Steam Eruption burn) or be KO'd. The coverage for Primarina and other Tera Fairy Pokémon doesn't hurt either.
Preferred Partners:
Sinistcha and Rillaboom immediately come to mind as partners for Volcanion, handling opposing Water and Electric types well (on top of Ground attacks) while greatly improving its durability with Hospitality/Grassy Terrain respectively. Volcanion for its part is a hard stop for opposing Fire types and Ice attacks, covering some of their weaknesses.
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