MM! Noted, thanks! Below are some calculations... Mind you it is not calculating in stealth rock dmg to the Volcarona for some reason, so the chance of 2HKO is probably much greater in the bulky case.
Timid Volcarona with 4 Def EVs:
~~~~+1 252 SpA Volcarona Bug Buzz vs. 0 HP / 0- SpD Garchomp: 288-340 (80.6 - 95.2%) -- 12.5% chance to OHKO after Stealth Rock
~~~~68 Atk Garchomp Earthquake vs. 0 HP / 4 Def Volcarona: 202-238 (64.9 - 76.5%) -- guaranteed 2HKO
but the garchomp outspeeds at +1
For a bulky Volcarona: Bold Nature