Umm... What the heck was that about up top?
Anyway, The counter is obvious!
Fire and Fighting.
This being so, Either works, but its easiest to just carry both, or a pokemon with a fire move.
Also, Ferrothorn may have crazy defenses, but his special defense is lower than his Defense, so a good Special Fire type move such as flamethrower will be able to easily wipe him out.
In other words, just have a Special Attacking pokemon , who has atleast 1 fire move, ( Flamethrower is my most reccomended ) And you should be good to go.