A major threat to your team as you said Volcarona. Volcarona, also is a major threat because although you can destroy it with Garchomp holding an attack, you loose Deoxys-D and this is not a good start to the battle for the team.
To solve the problem in a quick fix would be to make Volcarona Choice Scarf Gengar with Destiny Bond, but Volcarona will never be a team without a Rapid Spinner then we lose now and goodbye to delete Gengar Entry Hazards.
If you want a Pokemon that stops Volcarona, you're forced to replace a Pokemon, or at least make Garchomp with Choice Scarf Stone Edge
comes on and kills Quiever Dance Volcarona with Stone Edge, but at that point no longer work that appeals to you with Life Orb and Dragon Tail, but at least it saves you from Volcarona without damage.
So I would say try to see how it works with Choice Scarf and Stone Edge, it costs nothing, I tried to find a solution to Volcarona without making major changes because I see that every Pokemon has a role in this team. I hope to have helped. Good team! Luvdisc'd
Good luck, with your team!