Overall this team is pretty good, but there are a few changes I think you should make.
Tapu Koko: Pretty good, but Roost is a problem: Choice Scarf + Roost makes no sense since you're locked into it. I like the tactic with Lighting Rod + Discharge but with that, Volt Switch won't work (Sceptile will absorb it and prevent you from switching). Try U-turn instead and give it Max Attack IVs.
Sceptile: Energy Ball + Giga Drain is not very good; you only need one grass-type STAB. I recommend removing Energy Ball and replacing it with something like HP Fire. Why Bulldoze? If it's with Tapu Koko Bulldoze will do Huge Damage to Tapu Koko. I would probably replace it with Protect (useful in Doubles).
Metagross: Set is overall very good. I would recommend either some setup move like Agility or Bullet Punch for priority over Ice Punch, but you don't have to. Ice Punch is good for countering Ground, although it does work better if Metagross is Mega (with the Tough Claws boost). Don't make it Mega if you want the Mega Sceptile set.
Garchomp: Good set, just perhaps replace Earthquake with Stomping Tantrum (since Earthquake is a spread move and doesn't work as well in Doubles) or if you really want to trigger Metagross's Weakness Policy get Bulldoze and give Metagross Bullet Punch as well as max Defense EVs. Again, Protect would be nice here if you don't want to be damaged on that turn. Poison Jab is an option but since you'll be using it with Metagross I don't recommend it for this set.
Volcarona: I've never thought of a Quiver Dance / Fiery Dance + Dancer Oricorio set before, but it seems to be very smart. I don't know if Tailwind is necessary since Oricorio and Volcarona are both fairly fast. Again, try Protect in that slot. It's also missing an item; why not something like Buginium Z or Leftovers?
Oricorio: Seems decent to me; but why HP Ground? Roost can be used to regain health, and of course, Protect, which allows it to set up with Dancer while not taking damage.
Overall, this is a good team, but a few improvements are needed (don't forget Protect in Doubles XD) Hope I helped!
