Rotom-M @ Leftovers
252 SpAtk / 252 Spd /4 HP
Leaf Storm is the a great Grass-Type move, with that nasty SpAtk drop.
Confuse Ray + Thunder Wave is what my shiny Mow-Rotom has in my X game, and it has saved me in many WiFi battles, it is an annoying combo.
The last slot is really your choice. Thunder if you want power, Thunderbolt if you want accuracy, or Volt Switch to get out of there. Volt Switch is nice combo with Leaf Storm, that way you can get rid of the SpAtk drop when you switch out, but at the cost of power.
You can also replace Thunder Wave or Confuse Ray with Will-O-Wisp if you want to wear down health and cut their attack. You can also give Rotom the move Trick, then give your opponent a choice scarf, or another choice item.