I'd recommend using Bug Buzz over Energy Ball or Volt Switch on Galvantula. Secondary STABs are always good to have.
Volcarona doesn't need Fire Blast and Fiery Dance at the same time. That's redundant as both are Fire moves. Choose between one of them.
Jellicent's EV spread should be 248 HP/172 Def/88 Spe. This lets it outspeed Scizor and Bellyjet Azumarill among others.
Consider Flash Cannon on Hydreigon to let it deal with Fairies:
-Draco Meteor
-Dark Pulse
-Earth Power/Fire Blast
-Focus Blast/Flash Cannon
Nature: Modest
EVs: 24 HP/252 SAtk/232 Spe
Item: Choice Specs
I'd have to agree with this one above. However, you could do the ever-so-popular Life Orb set with Hydreigon, giving it dragon pulse rather than Draco Meteor, as to prevent the decrease in special attack. Fire Blast let's it deal with the mawhiles and lucarios, as well as major bug types. Flash cannon is almost NECESSARY on a Hydreigon set due to the Fairy type.
I found that a Leftovers Volcarona with Quiver Dance, Fiery Dance, Bug Buzz, and Giga Drain is a very useful moveset, able to dish out a ton of damage to popular Rotom-Washes and Mamoswines.
That would leave you to put a different item on Jellicent, which could be a sitrus berry or possibly a berry that allows it to halve damage from super-effective moves, because you already have use of your Recover move, which almost renders Leftovers useless.