Volcarona is part bug, so it doesn't get hurt SE with Earthquake, making your berry redundant. I would really prefer running Timid natured one with Life Orb or Leftovers.
I'd also adjust it's IVs -maybe even 29 HP IV so it'd hit 159 HP, making rocks/ spikes/ orb recoil smaller compared to what you're getting while having 160 HP. If you're running 31 HP IV I'd rather put the 4 EVs into HP. Passho berry makes a lot of sense since Gyarados is quite a common switch in for Volc. I'm just unsure about Giga Drain (I think I'd rather want HP Ice for Garchomp/ Dragonite).