Please rate my OU team.

Question

Nihilego @ Air Balloon
Ability: Beast Boost
EVs: 252 HP / 4 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Stealth Rock
- Toxic Spikes
- Toxic
- Clear Smog

Tyranitar @ Weakness Policy
Ability: Sand Stream
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Earthquake
- Crunch
- Rock Slide
- Fire Punch

Latios-Mega @ Latiosite
Ability: Levitate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Dance
- Dragon Claw
- Earthquake
- Zen Headbutt

Clefable @ Leftovers
Ability: Magic Guard
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Cosmic Power
- Wish
- Moonblast
- Stored Power

Greninja @ Life Orb
Ability: Protean
EVs: 4 Atk / 252 SpA / 252 Spe
Hasty Nature
- Hydro Pump
- Ice Beam
- Grass Knot
- Shadow Sneak

Heatran
Ability: Flash Fire
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Flamethrower
- Dragon Pulse
- Earth Power
- Ancient Power

Comments

Nihilego as a set-up pokemon just doesn't sit well with me.  I would switch it with a Ferrothorn or (if you really want Toxic Spikes) Forretress.  Either that, or try to make it into more of a tank/sweeper.
For Greninja, Water Shuriken is a Special Priority attack that you could use instead of Shadow Sneak.  Then you wouldn't need that attack investment.
If you want a suggestion for what to give your Heatran, I would suggest Assault Vest

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Thx! I forgot to give Heatran an item lol!

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Replace Nihilego and Latios with Toxapex for recovery and Medicham for better ability and offensive typing. Whatever you do, don't make Nihilego a tank. It has way to many weaknesses. Give Tyranitar choice band to hit hard without taking a super effective hit and risk fainting and 252 Atk/4 SpD/252 Spe to outspeed more walls. Replace rock slide with stone edge for more power. Give Clefable 252 HP/252 Def/4 SpD bold for more bulk. Replace cosmic power and stored power with stealth rock to lay hazards and protect to make wish more reliable. Give Greninja 4 Def/252 SpA/252 Spe timid for a little more bulk. Replace grass knot and shadow sneak with gunk shot to resist grass, bug, and fairy and spikes to resist electric. Give Heatran leftovers for healing and 4 HP/252 SpA/252 Spe timid to outspeed tanks like Tapu Bulu. Replace flamethrower, dragon pulse, and ancient power with toxic to break walls, magma storm to trap opponents, and taunt to stop status moves.