Yeah... ferrothorn is one of those Pokemon that are a little difficult to use, but can change the course of some battles.
If you want some ideas... here's a video of me successfully using ferrothorn: 84-09866-44175
This is what I used:

Ferrothorn @ Occa Berry
Brave(+Atk, -Spe) (0 Spe)
Iron Barbs
252 HP / 158 Atk / 100 Def
- Protect
- Leech Seed
- Gyro Ball
- Power Whip
Generally, the Ferrothorn can OHKO Rotom-W, offensive Politoed, and Gastrodon. It can also 2HKO T-Tar, take out MEtagross through leech seed stall, and work as a great check for rain. Gyro Ball can 2HKO most of the speedier threats like Latios and Garchomp. The thing is you probably should watch out for any SE moves, and avoid it from hitting ferrothorn, but outside of that, the rewards he can bring are great!

just making sure I have a good chance of getting more championship points.