Rotom-Wash is gonna be your man, as his one weakness is Grass, which your team can take with it's eyes closed, and he is immune to ground, and quite bulky.
Rotom-W @ Leftovers
EVs: 232 HP / 56 SAtk / 220 Spd
- Volt Switch
- Hydro Pump
- Pain Split
Hits fairly hard, has a lot of bulk, and access to STAB Bolt Switch and Pain Split, Rotom-W has been one of my favorite bulky attackers for a long time. Also note that it synergizes extremely well with your team. Also, you should replace Octillery.