Blissey @ Leftovers
Calm / 4 HP / 252 Def / 252 SpDef
Seismic Toss / Softboiled / Stealth Rock / Thunder Wave
Chansey @ Eviolite
Bold / 252 HP / 252 Def / 4 Spe
Seismic Toss / Wish / Protect / Toxic
So the two blob sisters are back again to bring hell to any and all special attackers that lack physical coverage. They can both use their massive SpDef and HP stats to sit and wall Volcarona all day, even in the Sun, and gradually chip away at its health with Seismic Toss or status moves, all the while using their quick recovery support moves to heal themselves. Blissey can tank even +4 Bug Buzz and not even get 2HKO'd- giving it plenty of time to chip away at Volcarona's health with Seismic Toss. Softboiled also makes Blissey a tough blob to smush. Blissey can also set SR, which discourages Volcarona from switching out because it won't be able to work as effectively the next time it comes in. Chansey is very similar- though it doesn't have the same SpDef bulk as Blissey does, it can Toxic stall with much more ease as it passes itself wishes and protects every other turn. It too can Seismic Toss Volcarona to death, but Flame Body burns are annoying.
It's probably useful to note that it takes 4 Seismic tosses to take down 100% 0 HP Volcarona.
Here are some calcs:
Fire Blast:
+3 252 SpA Volcarona Fire Blast vs. 252 HP / 0 SpD Eviolite Chansey: 322-379 (45.73 - 53.83%) -- 44.53% chance to 2HKO
+3 252 SpA Volcarona Fire Blast vs. 4 HP / 252+ SpD Blissey: 294-346 (45.09 - 53.06%) -- 0.39% chance to 2HKO
Bug Buzz:
+4 252 SpA Volcarona Bug Buzz vs. 4 HP / 252+ SpD Blissey: 264-312 (40.49 - 47.85%) -- guaranteed 3HKO
+4 252 SpA Volcarona Bug Buzz vs. 252 HP / 0 SpD Eviolite Chansey: 289-342 (41.05 - 48.57%) -- guaranteed 3HKO
Fire Dance:
+5 252 SpA Volcarona Fiery Dance vs. 252 HP / 0 SpD Eviolite Chansey: 301-355 (42.75 - 50.42%) -- 1.17% chance to 2HKO
+6 252 SpA Volcarona Fiery Dance vs. 4 HP / 252+ SpD Blissey: 313-369 (48 - 56.59%) -- 33.2% chance to 2HKO
@ Kidogo: the movesets can vary and they often do. Because what the other moves are doesn't really effect how well they counter Volc as long as recovery / seismic toss are on there, I just slapped on what I usually use on my blob. But honestly, it doesn't matter; it depends more on the rest of your team, which isn't what we're worrying about right now.
And no, Max SpDef Blissey has more SpDef than min SpDef Chansey with Eviolite, not the other way around. Chansey should be used when having mixed bulk is more of a prioirty, while Blissey should be used when Special bulk is valued more. It's not always dependent on weather of moveset.