BUZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ!!! GENERAL -Ever since its introduction in Generation V. It has been one of the best setup sweepers to have in OU. -135 Special Attack backed up by a respectable 100 Speed makes it the most threatening Quiver Dance user nuking its Special stats and Speed to heavenly heights. -Solid abilities in the form of Swarm and Flame Body. -The introduction of Talonflame and Mega Pinsir in Generation VI brought in a few hard stops to it due its low physical bulk. -4x weakness to Stealth Rock makes it heavily reliant on hazard control. -Heavily reliant on Quiver Dance boost to guarentee kills. FAST QUIVER DANCER Volcarona @ Lum Berry / Passho Berry Trait: Flame Body EVs: 72 HP / 252 SAtk / 184 Spe Timid Nature - Quiver Dance - Fire Blast / Flamethrower - Giga Drain - Bug Buzz / Hidden Power Ground / Hidden Power Rock This set is pretty straightforward. It aims to achieve as much kills as possible while becoming a menacing Pokemon to revenge kill via special attacks thanks to Quiver Dance. The set's speed EVs put you at 310 Speed which lets you outspeed Jolly Landorus-T naturally, meaning you outspeed Choice Scarf Landorus-T at +1. You can also opt for 252 SAtk / 4 SDef / 252 Spe if speed tying with other positive base 100s is a concern. Fire Blast (Flamethrower can be used if accuracy is an issue) at +1 is generally used to power through most Pokemon as it becomes a very lethal move against Pokemon that don't resist Fire type moves. Giga Drain makes the Moth able to take on bulky Water and Ground types for huge amounts of damage and also helps it recover lost HP. The final move can be used according to the Pokemon the team hasn't covered for yet. Bug Buzz is a secondary STAB and can power through Bulky Psychic types like Mew and Cresselia. It also hits Tyranitar harder than a Giga Drain. Hidden Power Ground can be used to take on Heatran which otherwise hard counters Volcarona on any other occasion. Hidden Power Rock gets a mention as it one shots any unlucky Mega Pinsir or Talonflame switching in. Do note that Mega Pinsir stills fears a Fire Blast / Flamethrower from Volcarona. Flame Body is the ability of choice as it can land a burn on many of Volcarona's wannabe revenge killers, notably Choice Band Dragonite and Mega Pinsir. The item choices vary from team to team. Lum Berry lets you continue a sweep even versus the likes of Prankster Thundurus with Thunder Wave and Discharge Zapdos. Passho Berry can be used as well as it helps in beating threats like Keldeo and Manaphy after a boost with Giga Drain. OTHER OPTIONS Items like Sitrus Berry or even Leftovers can be used to keep Volcarona healthy during its sweep especially when it's taking chip damage from weather and/or resisted priority moves like Bullet Punch, Mach Punch, and Vacuum Wave. Life Orb can be used to maximize damage output at the cost of losing health every hit. You can also opt for a Choice Specs set with Fire Blast (Or Overheat), Bug Buzz (Or HP Ground), Giga Drain and U-Turn for scouting out threats. You can also opt in for Hidden Power Ice as it threatens common switch-ins to Volcarona like Bulky Dragonite and Specially Defensive Mega Altaria. Fiery Dance is worth mentioning as well, it is 100% accurate as compared to Fire Blast and offers a 50% chance to boost Special Attack by a stage though the immediate isn't particularly appreciated. CHECKS AND COUNTERS Charizard X - Be it specially defensive or the Dragon Dance set. Charizard can switch into any of its moves bar Hidden Power Ground (Which it can still switch into when non Mega-Evolved). With it having a 4x resistance to its Fire type moves, it can keep roosting off of Volcarona's moves and keep dealing a considerable amount with its Physical moves due to its low physical bulk. Heatran and Talonflame - Heatran can easily come in to any of its moves bar HP Ground and proceed to deal damage to it via Ancient Power / Stone Edge or toxic poison you with Toxic or just phaze you out with Roar. Air Balloon can take its chances against Volcarona with HP Ground as well. Choice Band Talonflame and its other offensive variants check it a few times and threaten it with Priority Brave Bird but Specially Defensive Talonflame downright counters it. It is able to stop its setup by using Taunt or setting up on its own with Bulk Up with Roost for recovery makes Vocarona a setup fodder against it as it is immune to burns as well. Bulky Dragonite and Mega Altaria - Both can come in on Volcarona without any problem due to their solid special bulky combined with their typing that gives them resistances to both its types. Dragon Dance variants on both of the above mentioned Pokemon spells disaster for Volcarona as they make setup fodder out of it though they need to wary of being burnt via Flame Body. Do note that HP Ice is usable on Volcarona to stop the above two threats. Chansey - The blob can toxic poison, paralyze, or just downright hit you with Seismic Toss while recovering with Softboiled. Volcarona stands literally no chance even with a couple of boosts.
Sash puts immense pressure on your spinner/defogger though, because anyone seeing a Volcarona in the opponent's team preview will know they need their hazards up ASAP, to keep the bug in priority range. Not to mention Sandstorm, what with AssVest TTar being a pretty solid answer: +1 252 SpA Volcarona Bug Buzz vs. 248 HP / 0 SpD Tyranitar in Sand: 306-360 (75.9 - 89.3%). And that's without the vest.
Azumarill might be worth a mention, seeing as it resists both STABs, takes pittance from anything else bar Giga Drain, and can inflict solid damage with any Water STAB. Hell, itemless Aqua Jet is a guaranteed OHKO after Stealth Rock damage, assuming no Passho Berry. (Choice Band Aqua Jet is a clean OHKO regardless).