Heatran @ Leftovers / Grassium Z
Ability: Flash Fire
EVs: 212 HP / 44 SpA / 252 Spe (if Lefties) OR 252 SpA / 4 SpD / 252 Spe (If Grassium Z)
Timid Nature
- Flamethrower / Lava Plume / Magma Storm
- Toxic / Earth Power / Solar Beam
- Earth Power / Taunt /Stealth Rock
- Protect / Substitute
Heatran is one of the ultimate Ferrothorn killing machines. Even with practically no investment a Heatran can kill most Ferrothorns easily with a Lava Plume. For a Ferrothorn user, a Heatran is a free switch at best, and a seeded/paralyzed Heatran at worst.
Heatran is not particularly restrained on what moves you give it. You can Taunt a Ferrothorn to prevent it from using Seeds, hazards, or T.Wave. You can Sub to avoid Seeds, Knock Off, or T.Wave. You can just Lava Plume or Flamethrower to kill it, or set up Rocks if you're confident of a switch (provided you carry these moves).
Meanwhile, the most Ferrothorn can do is Bulldoze, but even then, Banded Bulldoze only OHKOes the Lefties set 25% of the time, and the offensive sets make Ferrothorn fold since it sacrifices the needed bulk to survive.