Jolteon. Modest, Specs. A little overkill, but it works. Stealth Rock support can take down the sturdy ability, which makes it a ton easier, and there is no need for priority.
252+ SpA Choice Specs Jolteon Hidden Power Fire vs. 252 HP / 168 SpD Ferrothorn: 340-404 (96.59 - 114.77%) -- 81.25% chance to OHKO
252+ SpA Choice Specs Jolteon Hidden Power Fire vs. 252 HP / 4 SpD Forretress: 672-792 (189.83 - 223.72%) -- guaranteed OHKO
252+ SpA Choice Specs Jolteon Thunderbolt vs. 224 HP / 32 SpD Skarmory: 582-686 (177.98 - 209.78%) -- guaranteed OHKO
252+ SpA Choice Specs Jolteon Hidden Power Fire vs. 248 HP / 8 SpD Scizor: 532-628 (155.1 - 183.09%) -- guaranteed OHKO
Under rain:
Ferrothorn cannot be OHKOed under rain by Jolteon. Bummer.
252+ SpA Choice Specs Jolteon Thunder vs. 252 HP / 4 SpD Forretress: 432-508 (122.03 - 143.5%) -- guaranteed OHKO
252+ SpA Choice Specs Jolteon Thunder vs. 224 HP / 32 SpD Skarmory: 734-866 (224.46 - 264.83%) -- guaranteed OHKO
252+ SpA Choice Specs Jolteon Thunder vs. 248 HP / 8 SpD Scizor: 342-403 (99.7 - 117.49%) -- 93.75% chance to OHKO
Even though Jolteon needs SR support, and cannot OHKO Ferrothorn in the rain, it does the job best. Damage Calculator