I recommend either Hydreigon or Zoroark

Hydriegon-Flamethrower, Dragon Pulse, Focus Blast, and Dark Pulse

Zoroark- Night Daze, Focus Blast, Flamethrower, and Nasty Plot

If you want switch out night daze for dark pulse, and they are pretty much the same in attack and special attack, so take your pick. (Truthfully i would go with zoroark :))

Replace Mean Eye with any strongish ghost move. I would get rid of Toxic too. Other then that i dont know. Is this going to be a wifi team?

Dude, to tell u the truth i wont put stat changing moves with any of my pokemon, only offensive moves. And if u want to know what type of offensive moves u wanna apply u gotta check the stats of ur pokemon. If its attack is higher than its sp.attack choose the strongest physical attacks it can learn, but if its the opposite choose the strongest sp.attacks it can learn. But the best types of sp.attacks and physical attacks are the ones that have a chance to put a status problem (poison, paralysis, etc.) to ur opponent. And there is this guy in youtube named lueroi, he said something very smart, u have to put balance in ur team, for instance if u have a pokemon with high attack but low defence u should cover that weakness by getting a pokemon with high defence. Thats how i make my party and it really works everyone i battled with until now i beat him/her.
Hope this helped.