A dumb yet decent counter against volcarona would be bulky charizard x
Charizard-Mega-X @ Charizardite X
Ability: Tough Claws
EVs: 100 HP / 248 Atk / 160 Spe
Jolly Nature
- Flare Blitz
- Earthquake
- Will-O-Wisp
- Roost
+1 216+ SpA Volcarona Bug Buzz vs. 100 HP / 0 SpD Charizard-Mega-X: 139-164 (43.1 - 50.9%) -- 3.9% chance to 2HKO
+1 216+ SpA Life Orb Volcarona Bug Buzz vs. 100 HP / 0 SpD Charizard-Mega-X: 181-213 (56.2 - 66.1%) -- guaranteed 2HKO
248 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 32 HP / 108 Def Volcarona: 312-367 (97.8 - 115%) -- 87.5% chance to OHKO
Even without any bulk you can take one
+1 216+ SpA Life Orb Volcarona Bug Buzz vs. 0 HP / 0 SpD Charizard-Mega-X: 181-213 (60.9 - 71.7%) -- guaranteed 2HKO
Charizard-Mega-X @ Charizardite X
Ability: Tough Claws
EVs: 100 HP / 248 Atk / 160 Spe
Jolly Nature
- Flare Blitz
- Earthquake
- Will-O-Wisp
- Roost
+1 216+ SpA Volcarona Bug Buzz vs. 100 HP / 0 SpD Charizard-Mega-X: 139-164 (43.1 - 50.9%) -- 3.9% chance to 2HKO
+1 216+ SpA Life Orb Volcarona Bug Buzz vs. 100 HP / 0 SpD Charizard-Mega-X: 181-213 (56.2 - 66.1%) -- guaranteed 2HKO
248 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 32 HP / 108 Def Volcarona: 312-367 (97.8 - 115%) -- 87.5% chance to OHKO
Even without any bulk you can take one
+1 216+ SpA Life Orb Volcarona Bug Buzz vs. 0 HP / 0 SpD Charizard-Mega-X: 181-213 (60.9 - 71.7%) -- guaranteed 2HKO