I think you guys are overrating Ferrothorn. He's bulky as all hell, yeah, but he's not particularly strong. Gyro Ball can be a real menace, to be sure, but Power Whip is inaccurate despite its very high BP, and Ferrothorn's attack of 287 isn't anything to write home about. Sure, he destroys water/ground mons, but Power Whip is really his only effective attack - Steel is a very shit attacking type. Grass doesn't have great coverage, and Leech Seed, while obnoxious, isn't terribly painful. Plus he's just complete dead weight against Sun Teams, and Rain teams have ways to handle him. Honestly, forget about killing Ferrothorn and leave him for last if you're on a rain team...or you can use a certain fiery moth that I'm going to give an extensive review to soon.
There's also Toxicroak for handling him on Rain Teams. Ferrothorn can't even hope to scratch him save the extremely rare Bulldoze variant, while Toxicroak threatens him with double leftovers recovery and a STAB Drain Punch. Toxicroak also has extreme use for fighting opposing rain teams, which Toxicroak walls to hell and back.