About time we did Ferrothorn.
I want to comment on the aesthetics first. Ferrothorn looks funny. Comment made.
Ferrothorn is a defensive titan. Its high bulk and excellent defensive typing make it an excellent wall. Ferrothorn also has a respectable attack stat to use as well, plus some nifty offensive moves like Power Whip and Gyro Ball. While a base 20 speed seems awful, this is actually a huge boon for Ferrothorn thanks to Trick Room and STAB Gyro Ball. In addition, Ferrothron has a wide array of support options, which include Leech Seed, Thunder Wave, and Stealth Rock.
Despite a heavy weakness to fire and a weakness to the ever populous fighting, Ferrothorn is a major presence.
Cactus Ball of Assistance
Nature: Careful
EVs: 252 defense, 156 hp, 100 special defense
@: Leftovers
Spikes/Stealth Rock
Thunder Wave/Toxic
Leech Seed/Stealth Rock/Gyro Ball
Power Whip/Gyro Ball
These kinds of sets are usually not my cup of honey-sweetened tea, but this one is very dangerous. Spikes is the better option, but Stealth Rock can be just as awesome. For status, Thunder Wave is the superior option because it cripples offensive threats, but Toxic can shorten any stall wars. Leech Seed provides a means to shorten the stall war while adding to Leftovers recovery. However, Stealth Rock can be used here instead, as can Gyro Ball. Speaking of which, Power Whip and Gyro Ball are your STAB moves in case you get hit by Taunt, or have nothing else to do. Remember, don't use Thunder Wave with Gyro Ball.
Cactus Ball is a Spiked Shield
Nature: Brave
EVs: 252 attack, 100 hp and special defense, 56 defense
@: Leftovers
Curse
Gyro Ball
Leech Seed/Explosion
Power Whip/Explosion
This isn't a standard Ferrothorn setup, but I'm a fan of defensive titans that can be used offensively. Unfortunately, Gastrodon is better at this job. What would make Ferrothorn better at this is if this thing got Synthesis or something. But it didn't. Curse is the boosting move of choice with Gyro Ball being your go-to STAB move. Leech Seed can keep Ferrothorn going longer and Power Whip is an alternative STAB option. either of these moves can be dropped for Explosion, but that isn't usually the better way to go. If you want to treat Ferrothorn like C4, better replace Leech Seed.
Cactus Flail
Nature: Adamant/Brave
EVs: same as above
@: Choice Band
Gyro Ball
Power Whip
Bulldoze
Explosion
This set is seen from time to time, but I don't like it. Ferrothorn's defensive potential is not being used to the maximum with a set like this. But, like I always say: "If it ain't broke, don't fix it."
Other options:
Gravity is one of the big ones. Not only does it help with Power Whip's less-than-perfect accuracy, it can be a huge boon for your team as well.
Knock Off is another great move Ferrothorn can make great use of.
Worry Seed can be a ton of fun as well.
Ingrain might seem like a good idea, but Leech Seed is better.
Swords Dance might seem like a good idea as well. However, Ferrothorn isn't designed for sweeping. Curse is a better boosting move for the cactus ball.
Countering:
Fire. Have a lot of fire. You should anyway. Having something on your team with at least Flamethrower will go a long way. Try to avoid Iron Barbs by using Special Attacks if you can.