The first thing I notice when looking at this team is that it's incredibly fragile. Imagine you are facing a Dragonite or Mega Altaria and it gets to set up a Dragon Dance. You have no way to stop it. It will sweep your whole team. You should have a ravenge killer (a strong priority user or a choice scarfer) to stop being swept, or a physical wall like Skarmory or Unaware Quagsire.
The second mistake I notice is that you are playing too much setup. You normally carry in your team a single setup sweeper (maybe 2) and you build the team around it in order to make it your winning condition. But you are playing two Swords Dances, a Quiver Dance and a Calm Mind. That's way too much. You may for instance want to give a support role to Sylveon, with moves like Heal Bell or Wish. You have three physical attackers that don't enjoy being burnt, and a paralysis in Volcarona or Starmie makes them fairly useless, so some way of healing status would be good for the team.
Finally, some little details: Jolly over Adamant for Gallade, and Pixilate Hyper Voice over Cute Charm Moonblast on Sylveon. I also prefer Knock Off over Zen Headbutt and Will-o-Wisp or Thunder Wave over Swords Dance in Gallade, in a more supportish role. For Volcarona, I find Fiery Dance preferable to Fire Blast for two reasons. First, missing a critical Fire Blast can make you lose, and with 85% accuracy it happens, believe me. Second, Fiery Dance is actually stronger than Fire Blast after a boost.
The best way of improving a team is by actually playing it so do go to play.pokemonshowdown.com and test the team. That's the best way of spotting its weaknesses.
Taking into account the team's many weaknesses, I would suggest you to chise the two or three pokemon you don't want to change, and start from scratch with the rest of the team, trying to cover the weaknesses of the core.
