i OU :)

Apollo (Mega-Charizard Y):
Ability: Drought
Nature: Timid
EVs: 4/0/0/252/0/252
Moves: Fire Blast/Flamethrower (FB in sun is amazingly strong), Solar Beam, Air Slash, Ancient Power (4x to Talonflame and other Charizards, Mega-Y has same speed as normal and protect can be set up on easily)
Item: Charizardite Y (duh)

IAmGroot (Trevenent):
Ability: Harvest
Nature: Careful
EVs: 252 HP, 48 Attack, rest in Sp. Def (204?) - WoW will do enough to absorb physical damage so focus on Sp.Def
Moves: Substitute, Will-O-Wisp, Leech Seed, Phantom Force/Horn Leech - Phantom Force is useful for getting an extra turn to harvest your Sitrus berry, Horn Leech if you detest the idea of using Phantom Force
Item: Sitrus Berry


RougeLegacy (Greninja):
Ability: Protean
Nature: Timid
EVs: 4/0/0/252/0/252
Moves: Ice Beam, Scald, Grass Knot, Dark Pulse
Item: Life Orb


Robocop (Metagross):
Ability: Clear Body
Nature: Impish
EVs: 172/168/168/0/0/0
Moves: Bullet Punch, Earthquake, Stealth Rock, Zen Headbutt
Item: Leftovers (nothing changed)


PixieDust (Sylveon):
Ability: Pixilate
Nature: Bold
EVs: 172/0/252/0/80/0 - You'll be boosting your special defense & special attack with Calm Mind, so make sure to bolster your physical defense to be able to take hits.
Moves: Calm Mind, Hyper Voice, Stored Power, Wish
Item: Lum Berry




Finn (The Human! just kidding! It's a Garchomp):
Ability: Rough Skin
Nature: Jolly
EVs:4/252/0/0/0/252
Moves: Swords Dance, Dragon Claw, Earthquake, Rock Slide (Brick Break doesn't give you any coverage more useful or outdamaging than a +2 Earthquake/Dragon Claw/Rock Slide.
Item: Life Orb

 

Instead of Metagross, I'd recommend Espeon
Ability: Magic Bounce
Nature: Timid
EVs: 252 Sp.atk/252 Sp/4 Hp
Moves: Light Screen, Reflect, Psyshock, dazzling gleam
Item: light clay

Your team is left open to hazards/ status moves so magic bounce would work well as a counter for those.

I'd also recommend this.
RougeLegacy (Greninja):
Ability: Protean
Nature: jolly
EVs: 252 Atk/252 Spd/4/hp
Moves: U-turn, Shadow sneak, Waterfall, and Gunk Shot
Item: Life Orb

This physically attacking Greninja catches everyone off guard. Greninja is a mixed attacker so this works well. Shadow sneak is for priority while the other moves give different type coverage than the other Greninja.

 

Spoiler

[3D='greninja']
RougeLegacy (Greninja):
Ability: Protean
Nature: Naive
EVs: 4 Atk/252 SpA/252 Spd/
Moves: Hydro Pump, Ice Beam, Dark Pulse, Gunk Shot
Item: Life Orb

Gunk Shot lets you OHKO pretty much any Faiiry. 'nuff said

Also, move Stealth Rocks onto Garchomp (over Rock Slide) and use Swords Dance/Fire Blast over EQ.

P.S. Your team dies vs. Greninja and SD Talonflame, it would be nice to have a check to those 2. I'm not sure what I'd recommend though, tbh. Maybe replace Trevenant and Metagross with another defensive core? Like Heatran and something else.

You could also put Rocks on Heatran + Choice Scarf Garchomp. I dunno.

 

This is without touching the Pokemons, now if you are open to change some members on it, I will put other suggestions.

Spoiler

Charizard-Mega-Y (M) @ Charizardite Y
Ability: Drought
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Flamethrower
- Dragon Pulse
- Solar Beam
- Air Slash

Greninja (F) @ Expert Belt
Ability: Protean
Shiny: Yes
EVs: 68 Atk / 188 SpA / 252 Spe
Hasty Nature
- Scald
- Ice Beam
- Rock Slide
- Grass Knot

Garchomp (M) @ Choice Scarf
Ability: Sand Veil
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Outrage
- Earthquake
- Rock Slide
- Fire Blast

Trevenant (F) @ Sitrus Berry
Ability: Harvest
Shiny: Yes
EVs: 172 HP / 168 Atk / 168 SpD
Adamant Nature
- Horn Leech
- Shadow Claw
- Substitute
- Will-O-Wisp

Metagross @ Metagrossite
Ability: Clear Body
Shiny: Yes
EVs: 252 Atk / 252 Spe
Jolly Nature
- Bullet Punch
- Meteor Mash
- Earthquake
- Zen Headbutt

Sylveon (M) @ Lum Berry
Ability: Cute Charm
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Hyper Voice
- Hidden Power [Ground]
- Calm Mind
- Wish

2 Suggestions that you can take if you want:

Spoiler

* If you make changes on the team try to show us the changes that you have done puting them in the main post. With this you allow the people that is arriving to know where start helping

* Try building the team in Showdown and export it to here . So we can import it to our PC and start studying it. This will allow to work faster on the process of improve your team.