Ferrothorn @ Leftovers
Ability: Iron Barbs
Level: 50
EVs: 252 HP / 52 Def / 204 SpD
Sassy Nature
IVs: 0 Spe
- Gyro Ball
- Power Whip
- Leech Seed
- Stealth Rock/Protect
Ferrothorn absolutely MISHES on Mega Swampert, taking like nothing from most of its moves bar Superpower, which it still lives, while obliterating it with a 4x super effective Power Whip.
252+ Atk Mega Swampert Waterfall vs. 252 HP / 52 Def Ferrothorn in Rain: 48-57 (26.5 - 31.4%) -- 16.3% chance to 4HKO after Leftovers recovery
252+ Atk Mega Swampert Earthquake vs. 252 HP / 52 Def Ferrothorn: 79-94 (43.6 - 51.9%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Mega Swampert Superpower vs. 252 HP / 52 Def Ferrothorn: 128-152 (70.7 - 83.9%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Ferrothorn Power Whip vs. 4 HP / 0 Def Mega Swampert: 240-288 (136.3 - 163.6%) -- guaranteed OHKO
The other moves are fairly standard. Gyro Ball is one of Ferrothorn's most spammable STAB moves, Leech Seed heals Ferro even further while chipping down more defensive switchins, while either Stealth Rock or Protect can be run in the last slot, with Protect being nice for chipping with Leech Seed or stalling rain turns if you already have a Stealth Rock setter. EV-wise, Ferrothorn has enough HP and SpD to 1v1 Sash Lead Pelipper, 3HKOing with Power Whip while being 4HKOed by Hurricane, with everything else dumped into Defense.
252+ SpA Pelipper Hurricane vs. 252 HP / 204+ SpD Ferrothorn: 57-67 (31.4 - 37%) -- guaranteed 4HKO after Leftovers recovery
0 Atk Ferrothorn Power Whip vs. 252 HP / 4 Def Pelipper: 64-76 (38.3 - 45.5%) -- guaranteed 3HKO