[Sweep]

Lustrous Orb Palkia can do a good job of putting it in mortal danger. Palkia can Spacial Rend or Hydro Pump Ferrothorn as Kyogre runs from it, then proceed to Fire Blast it. From there, as Ferrothorn hits Palkia with Leech Seed, switch out as a Protect is likely to follow.

Though most teams have a better SpecsOgre check than Ferrothorn, Specs can 2HKO it cleanly with Water Spout, ensuring it cannot switch in without paying dearly.

If you are really desperate, Magzezone can trap it and set up on it with a Charge Beam/Substitute combo, which Ferrothorn cannot break.

Choice Band Kabutops takes a massive chunk of damage out of it with Low Kick, especially after entry hazards. Apart from that, Ferrothorn is very difficult to deal with. Just keep hitting it with powerful attacks and play around the protect instead of staying in and letting it replenish its health.