Firstly, welcome to the Battle Center section of the forums!
Secondly, let's review your team. Each pokemon should display their EV spread and natures as well as movesets, abilities and items.
Your team is weak to Ice, Flying and Psychic-type moves. A Steel-type helps greatly in terms of defensive synergy, as they cover your team's weaknesses. Common Steel types include Heatran, Metagross, Ferrothorn and Scizor, so you could pick any of these if you like. Replace Scyther with it, because Scyther is never seen very often due to its massive Stealth rock weakness and limited coverage. Ferrothorn/Heatran are the preferred options as they can set up entry hazards such as Spikes and Stealth Rock, which can greatly help offensive Pokemon.
Here's two sets for Heatran and Ferrothorn respectively
~Fire Blast
~Earth Power/Flash Cannon
~HP Ice/Grass
~Stealth Rock
Item: Leftovers
Nature: Timid
EVs: 252 SpA / 4 SpD / 252 Spe
~Lava Plume
~Protect
~Roar
~Stealth Rock
Item: Leftovers
Nature: Calm
EVs: 248 HP / 8 Def / 252 SpD
-----
~Spikes/Stealth Rock
~Leech Seed
~Power Whip
~Gyro Ball
Item: Leftovers
Nature: Relaxed
EVs: 252 HP / 48 Def / 208 SpD
IVs: 0 Speed
~Gyro Ball
~Power Whip
~Bulldoze
~Spikes
Item: Choice Band
Nature: Brave
EVs: 248 HP / 252 Atk / 8 SpD
IVs: 0 Speed
Hyper Beam is never used on any Pokemon let alone Sylveon. I'd utilize another coverage move such as Shadow Ball or a set-up move like Calm Mind to replace HB.
Flygon doesn't have good enough defenses to make use of Dragon Tail. Outrage is the preferred option instead. Choice Scarf would be my item of choice for him.
Hope this review helps. Good luck!
For starters, Heatran isn't available or even legal in X/Y yet. Wait till Poke Transfer before even editing it in. Make do with Aegislash:
-King's Shield
-Shadow Ball/Iron Head
-Shadow Sneak
-Iron Head/Sacred Sword
Nature: Quiet/Adamant
EVs: 252 HP/4 Atk/252 SAtk or 252 HP/252 Atk/4 Def
Item: Leftovers
or
-Swords Dance
-Shadow Sneak
-Iron Head/Sacred Sword
-King's Shield
Nature: Adamant
EVs: 252 HP/252 Atk/4 Spe
Item: Leftovers
or
-Shadow Ball
-Shadow Sneak
-Iron Head
-Sacred Sword
Nature: Quiet/Brave
EVs: 4 HP/252 Atk/252 SAtk
Item: Spooky Plate/Life Orb
or
-Autotomize
-Shadow Ball
-Hidden Power (Ice)
-Iron Head/Sacred Sword
Nature: Rash
EVs: 88 Atk/252 SAtk/168 Spe
Item: Life Orb
or
-Swords Dance
-Shadow Sneak
-Shadow Claw
-Iron Head
Nature: Adamant
EVs: 252 HP/252 Atk/4 Spe
Item: Leftovers
Also, Ferrothorn can make better use of different EV spreads: Relaxed with 252 HP/200 Def/56 SDef or 252 HP/76 Def/180 SDef for more physical bulk or more balanced bulk, respectively.
Remember to play to Sylveon's strengths, such as its great special bulk:
-Wish
-Protect
-Moonblast
-Shadow Ball/Hidden Power (Fire)
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
or
-Calm Mind
-Moonblast
-Shadow Ball
-Hidden Power (Fire)
Nature: Modest
EVs: 252 HP/252 SAtk/4 SDef
Item: Leftovers
Scizor kinda outclasses Scyther because it hits harder. If you want to surprise opponents by using Scyther (it even has stats matching up to fully evolved Pokemon despite being a pre-evolved Pokemon), be my guest:
-Swords Dance
-Aerial Ace
-X-Scissor/Quick Attack
-Brick Break/Quick Attack
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Lum Berry
Ability: Technician
or
-U-turn
-Aerial Ace
-X-Scissor/Quick Attack
-Brick Break/X-Scissor
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band/Choice Scarf
Ability: Technician
or
-Swords Dance
-Aerial Ace
-Brick Break/Baton Pass
-Roost
Nature: Jolly
EVs: 248 HP/8 Atk/252 Spe
Item: Eviolite
Ability: Technician
Otherwise, use Scizor:
-U-turn
-Bullet Punch
-Brick Break
-Pursuit/Quick Attack
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Choice Band
Ability: Technician
or
-Swords Dance
-Bullet Punch
-X-Scissor
-Brick Break
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Life Orb
Ability: Technician
or
-Swords Dance
-Bullet Punch
-X-Scissor
-Roost
Nature: Adamant
EVs: 252 HP/12 Atk/244 SDef
Item: Scizorite
Ability: Technician
Toxicroak is only good in Rain teams. Lucario is the better fit outside of those teams:
-Swords Dance
-Close Combat
-ExtremeSpeed/Bullet Punch
-Crunch/Shadow Claw
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Lucarionite
or
-Nasty Plot
-Aura Sphere
-Flash Cannon
-Shadow Ball/Dark Pulse/Vacuum Wave
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Lucarionite
or
-Agility
-Close Combat
-Hidden Power (Ice)
-Flash Cannon/Dark Pulse/Shadow Ball
Nature: Rash
EVs: 252 Atk/84 SAtk/172 Spe
Item: Lucarionite
There's literally no reason to use Flygon in standard. Sure, it's fast, but it doesn't hit hard enough. Garchomp is the better option in the standard environment:
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Fire Fang/Stone Edge
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Lum Berry/Yache Berry/Haban Berry
or
-Outrage
-Earthquake
-Fire Fang/Stone Edge/Fire Blast
-Dragon Claw/Dual Chop
Nature: Jolly/Naive
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band/Choice Scarf
I don't like having Drifblim It doesn't have much to offer when other Ghosts hit faster or harder than it, like Gengar or Aegislash. Aegislash was already explained above.
Gastrodon lost its effectiveness in the generation shift, due to the rain nerf, but it's still a good check to opposing bulky Waters. It's better to use a Ground STAB instead of Toxic + Protect. Sure, Gastrodon can do that, but there are other Pokemon who can do it better, like Gliscor:
-Scald
-Earthquake
-Ice Beam/Toxic
-Recover
Nature: Relaxed
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Storm Drain