Earlygame, you'll probably be using Growth + Leaf Tornado and Leech Seeding. Use Return once you get it for some coverage, because too many Plasma grunts use Poison mons like Zubats and Trubbish. Later run Coil because it is superior to Growth and you'll most likely be only running physical attacks. Since you are playing BW2, you can tutor it Aqua Tail, and Coil will negate its imperfect accuracy and boost it. In the end I had:
>Coil
>Leaf Blade
>Return
>Aqua Tail
Nature doesn't matter too much. You'll be transitioning from mixed (mostly special) to physical later, just as long as it isn't a speed or attack hindering nature. Even + SpDef nature is fine because Coil raises Atk/Def/Accuracy. Snivy for in game is faster than nearly everything, if not everything, anyways. If you do well, you can sweep the entire Elite 4 + Champion with just Serperior. It's a little wonky to use to attack immediately, but being tanky, able to set up, and deal damage, it's superior for boss battles and certain trainers.
