I'm fairly new to this and have been lurking Pokemondb a while unregistered. I know X/Y came out recently but seeing the movesets offered for this Pokemon, I was disappointed that no one mentioned Forest's Curse and I just had to register an account (finally) just to offer it for anyone whose interest I may catch with this moveset idea.
Trevenant @ Sitrus Berry
EVs: 118 HP / 140 SDef / 252 Atk
Brave Nature (+Atk -Spe)
-Phantom Force/Shadow Claw/Wood Hammer
I chose Trick Room to take advantage of his good attack and poor speed. Your opponent may not expect it and it will (ideally) irritate them.
The most exciting part of it all is the Forest Curse and X-Scissor/Poison Jab combo. Forest Curse adds grass typing to your opponent. I think it's amazing that he can learn X-Scissor and Poison Jab, which we all know is super-effective to grass types. You can take your pick on which one you'd prefer, Poison Jab for 30% chance to poison your target (with the exception of Steel types though...), or X-Scissor.
And then you can take your choice for STAB: Phantom Force (goes through Protect and Detect), Shadow Claw (a solid choice), or Wood Hammer (very powerful, only downfall is the recoil). I personally prefer Phantom Force because protect is really irritating sometimes, so this gives the chance to be just as annoying to your opponent.