It depends on your moveset though, but as you already have a special wall, you can go with
Trait: iron barbs
IVs: 0 spe
Relaxed(if rocky helmet)/brave(if leftovers)
-seed bomb/power whip
Curse is to boost the attack and defense and to cut speed which makes gyro ball hit even harder. Seed bomb if you want accuracy and power whip for power. Leech seed for recovery.
This is a good set for ferrothorn if you don wanna invest in both defenses. And whether it is advisable to invest in attack or not depends on the rest of your team.
My ferro is defensive and special defensive because I have too many physical attackers on my team.
Hope I helped :)