I will say, though, that a lot of teams people bring can be pretty cleanly 6-0'd by Volcarona if it gets set up. Examples below from OUPL finals, just because they're recent:
https://replay.pokemonshowdown.com/smogtours-gen7ou-383947
Hiye's team - Clef/Heatran/Zygarde/Latios/Ferrothorn/Landorus - they all die to +1 Volcarona with Bug-Z and HP Ground. It's especially nice if Clef's taken just a tiny bit of damage so Volc can avoid taking a Twave. A teammate like Torn-T can taunt mons like Clefable, Heatran, or Ferrothorn, clear hazards, and let Volcarona in for free.
FMG's team - Lando/Lopunny/Zygarde/Rotom-W/Magearna/(CB) Kartana - they also die to +1 Volcarona with Bug-Z and HP Ground (Lopunny can try to RK with repeated Fake Outs, I guess, but risks catching a burn.
https://replay.pokemonshowdown.com/smogtours-gen7ou-383736
Vai Lusa's team is super similar to Hiye's
You can see similar scenarios for other teams in the finals, though cores like Gastrodon+Heatran are somewhat harder for Volcarona to break, these calcs bears repeating:
+1 252 SpA Volcarona Savage Spin-Out (175 BP) vs. 248 HP / 252+ SpD Gastrodon: 360-424 (84.7 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
+1 252 SpA Volcarona Hidden Power Ground vs. 248 HP / 220+ SpD Heatran: 288-340 (74.8 - 88.3%) -- guaranteed 2HKO after Leftovers recovery
If you somehow have spikes up, you win against either.
(e.g. https://replay.pokemonshowdown.com/smogtours-gen7ou-383770)
Hiye's team - Clef/Heatran/Zygarde/Latios/Ferrothorn/Landorus - they all die to +1 Volcarona with Bug-Z and HP Ground. It's especially nice if Clef's taken just a tiny bit of damage so Volc can avoid taking a Twave. A teammate like Torn-T can taunt mons like Clefable, Heatran, or Ferrothorn, clear hazards, and let Volcarona in for free.
FMG's team - Lando/Lopunny/Zygarde/Rotom-W/Magearna/(CB) Kartana - they also die to +1 Volcarona with Bug-Z and HP Ground (Lopunny can try to RK with repeated Fake Outs, I guess, but risks catching a burn.
https://replay.pokemonshowdown.com/smogtours-gen7ou-383736
Vai Lusa's team is super similar to Hiye's
You can see similar scenarios for other teams in the finals, though cores like Gastrodon+Heatran are somewhat harder for Volcarona to break, these calcs bears repeating:
+1 252 SpA Volcarona Savage Spin-Out (175 BP) vs. 248 HP / 252+ SpD Gastrodon: 360-424 (84.7 - 99.7%) -- guaranteed 2HKO after Leftovers recovery
+1 252 SpA Volcarona Hidden Power Ground vs. 248 HP / 220+ SpD Heatran: 288-340 (74.8 - 88.3%) -- guaranteed 2HKO after Leftovers recovery
If you somehow have spikes up, you win against either.
(e.g. https://replay.pokemonshowdown.com/smogtours-gen7ou-383770)
Disclaimer: This isn't to say "haha look Volc's good because it can theoretically win against OUPL teams" but rather that as the meta has developed, there are a few holes that Volcarona can still exploit, and I think it's worth mentioning them, even if Volc is actually destined for A-.
Also thisTLDR: HEATRAN S, ZYGARDE S, KARTANA A+, REMOVE S-.