This is possibly the best starter Pokemon in the game, up there with Blaziken. He's a sweeper and a utility all in one, and Protean allows him to beat whatever he may need to.
Greninja @ Life Orb
Ability: Protean
EVs: 120 Atk / 136 SpA / 252 Spd
Naive Nature
- Hydro Pump
- Hidden Power Fairy / Ice Beam
- Power-Up Punch
- Water Shuriken
The goal with this set is to take full advantage of Protean and forgo some of the utility moves like U-turn, Rapid Spin and Spikes. Bring him in, use a move like Hidden Power Fairy to change your type into something that would easily beat a potential switch in, and then play accordingly. Ice and Fairy type are weak to Steel, so opponents may try to ruin your day with Bullet Punch. That's where Water Shuriken comes in, as it can go first and promptly change you back to a Water type so you gain Steel resistance. It also isn't bad as a general priority attack that can finished off weakened opponents. Power-Up Punch would be best if Ferrothorn is giving you issues, as Greninja can become a Fighting type, deal damage, and gain an attack boost all in one go. Power-Up Punch also constantly increases the threat of Water Shuriken as you stack boosts, giving you a better chance at a sweep if your opponent cannot answer it in time. Switching between Water and Fairy type can cause some headaches, as the two types are very synergistic. I can't really say for sure if this is the right combination of moves just yet as I have not had the chance to test Greninja, but the idea is there and I think something similar could be a serious challenge to overcome.
All in all, this is an awesome Pokemon who I'd be surprised to see in any tier lower than OU.