You have 1 Defensive mon and 5 Offensive. You should go either Hyper Offense (six offensive) or Balance (3 offensive, 3 defensive). I recommend Balance (because I hate Stall with a burning passion, no offense stall players, and Balance is easier/more likely to be effective for most people) with these changes:
Vaporeon
Keep Vaporeon away from this team. It has too little stats to be usable in OU (and will get absolutely destroyed 90% of the time). Just pretend he/she is your mascot cheering from the sidelines, not getting killed in the arena, and thus having a better time than your team, which is currently getting horrendously swept by Zygarde :P.
(Just jesting. Don't take that seriously. You are more likely swept by Ash Greninja...)
If you want a Cleric, I recommend Tapu Lele or Chansey. Here are possible sets:
Chansey @ Eviolite
Ability: Natural Cure
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Aromatherapy
- Counter/Seismic Toss
- Soft-Boiled
- Healing Wish/Toxic
Counter often hits harder than Seismic Toss, but requires better prediction and can only be used against physical attackers. Healing Wish is to be used if you want to fix up a teammate, or if you choose Toxic, it wears down many non Steel/Poison types.
Tapu Lele @ Leftovers
Ability: Psychic Surge
EVs: 252 HP / 232 Def / 16 SpA / 8 SpD
Bold Nature
IVs: 0 Atk
- Aromatherapy
- Hidden Power [Ice]/Hidden Power [Ground]
- Psyshock/Psychic/Moonblast
- Toxic/Psyshock/Psychic/Moonblast
Fire is already covered by Greninja, so HP Ice (for Landorus Lure, because most switch out of Greninja) or HP Ground (For Heatran Lure) could work. If you want to use HP Ground run this spread instead: 252 HP, 252 SpD, 4 SpA, Modest/Calm Nature. Aromatheropy is the same thing as Heal Bell. Psychic/Psyshock/Moonblast is up to you, but I use Psychic/Psyshock for Psychic Terrain's Psychic damage boost. You can have Toxic or another damaging move as well.
Garchomp
Scarf-chomp is way overrated, even for my liking. I suggest Rocks Setter Garchomp:
Garchomp @ Leftovers
Ability: Rough Skin
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Dragon Claw
- Earthquake
- Fire Blast
- Stealth Rock
If you want to use Fire Fang instead of Fire Blast, use this spread: 252 Atk, 252 Spe, 4 SpD, Jolly Nature, just don't come crying to me when your Garchomp gets walled by Skarmory, which then sets up 3 Spikes and Counters Garchomp to death. As said, Fire Blast hits Skarmory better (and Mega Scizor, but Sciz generally doesn't survive HP Fire so...), and hits Celesteela and Kartana (if you survive Kartana, somehow) just as good as Fire Fang.
Bisharp
Bisharp isn't bad, when used correctly. Bisharp is usually a Z-Nuker, but since you have Z-Mimikyu, it is bets to dump him and use Mew or Landorus:
Mew @ Leftovers
Ability: Synchronize
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Defog
- Will-O-Wisp/Transform
- Earthquake
- U-turn
Mew is suggested because adding Landorus will make your team more weak to Ice, which is not advised when Greninja and Mega Latios amost always pack Ice Beam. Will-O-Wisp hinders physical attackers, while Transform can be used... but not suggested since you can't (and probably shouldn't) use Normalium Z. Defog gets rid of those nasty Entry Hazards, Earthquake is good coverage, and U-Turn is a reliable pivot. You could use this spread if you want Mew to survive more hits rather than attack better: 252 HP, 20 Atk, 68 Def, 168 SpD, Impish Nature (use You may use Swords Dance instead of Will-O-Wisp and Knock Off instead of U-Turn with this spread).
Landorus-Therian (M) @ Leftovers
Ability: Intimidate
EVs: 252 HP / 252 Atk / 4 SpA
Naughty Nature
- Defog
- Earthquake
- Hidden Power [Ice]/Knock Off/Superpower
- U-turn
This Landorus-T Can help a lot; it can use Intimidate to weaken Physical Attackers, HP Ice OHKOs other Landorus, U-Turn is a reliable Pivot, and Earthquake is good STAB. If you don't use Hidden Power, use this spread: 252 HP, 252 Atk, 4 Def, Adamant Nature or 252 HP, 252 Def, 4 Attack, Impish Nature. Knock Off cripples item abusers and Superpower OHKOs stuff like Chansey.
Mimikyu
Edit: Ghostium Z is good, as pointed out by sumwun, but if you don't want it to use a Z-Crystal, use a Life Orb.
Greninja
Greninja is fine.
Mega Venasaur is fine.
I feel half bad for Vaporeon, not being used by someone who apreciates it, but also half not bad, because the other Pokemon in OU would demolish the little thing. Its for Vaporeon's own good it doesn't compete in OU.
Hope you have a wonderful time in OU. The only way to see if a team works is to practice (by battling/test battling) and know how each member works, discover your team's core, know the metagame, and tweak anything that could be better! :P
Hope I helped!
