I have 3 different Ferrothorns. Each have a different role. That's how much I love this guy.
Stall defensive ferrothorn(Warning: Do not go up against any steel types when using this moveset.)
Toxic: A necessity move for most stall Pokemon.
ingrain: Heals ferrothorn's hp a little each turn.
Leech seed: Steals a little health from the opponent each turn.
Explosion:Oh no my ferrothorn is about to faint. Might as well commit suicide!!! :D
Strategy:The strategy here is pretty self explanatory. You have toxic which takes more and more health from your opponent each turn. Then using ingrain will make ferrothorn gain health each turn. Then leech seed will steal some health from your opponents Pokemon each turn. Finnally when all else fails and your stalling strategy comes to an end commit suicide using explosion!!! If I die you die with me!!!
Mix attack and defensive ferrothorn:
Gyro ball: works well with that low speed ;D
Power whip:A powerful physical grass type move
shadow claw: Good move with decent power and high critical hit ratio.
Curse:Attack and defense raises but speed lowers. There are two reasons why this is good however: 1: Its speed sucked to begin with and 2: It powers up Gyroball.
Strategy: Curse will power up pretty much everything. Your attack, defense, and your move gyro ball. Shadowclaw is for incase you come across steel types. (its not super effective against them but it does do normal damage. Not to mention it can be used against other Pokemon.)
Physical sweeping Ferrothorn:
Power whip: Strong physical grass move.
Gyro ball: The slower you are compared to the opponent the greater the damage.
Giga impact/shadowclaw/poison jab/bulldoze: Giga impact is a very powerful move but you must recharge for one turn. Shadowclaw has decent power and a good critical hit ratio. Poison jab is a physical poison type move with good power. Bulldoze will not only do damage but will also lower your opponents speed and is good on steel types.(though sadly this will decrease gyro balls power, you can however replace gyro ball with iron head. The choice is up to you). I personally would give mine shadowclaw but like I said before its up to you.
Swords dance/Explosion: Either faint yourself along with your opponents if your gonna faint anyways or increase your attack even further.
Strategy: Unlike the other Ferrothorns. This one is a full offensive physical sweeper. Gyro ball is used for slow users. Power whip like I said previously is a strong physical grass move. Give it another physical move of your choice. .Finally You can either give it swords dance (which will strongly increase its attack.) or give it explosion (incase your Ferrothorn is almost dead.)