Camerupt @ Cameruptite
Ability: Magma Armor / Solid Rock
EVs: 128 HP / 252 SpA / 128 Spe
Modest Nature
- Fire Blast
- Earth Power
- Nature Power
- Stealth Rock / Rock Slide / Ancient Power / HP Whatever / Toxic / BlahBlahBlah
Camerupt easily OHKOes both members of the core with Earth Power and Nature Power into Energy Ball (yes this requires bulu support but if you really want go run HP Grass). Can't switch-in to Gastrodon but can take an Earthquake. It can take the Magma Storm from Heatran pretty easily and Earth Power does around 50% so you can switch in if needed. Apart from threatening both members it can take the opportunity to set up Stealth Rocks or to fire off a strong attack (as a bonus it threatens grass type Pokemon that can be paired with the core to form a Fire-Water-Grass core).
Will post calcs later