You need to run leech seed because it damages your opponent, and it allows you to switch out and heal some if your other pokemon. Here is the moveset I would run.
Item Attached: Rocky Helmet
Ability: Iron Barbs
EVs: 252 HP / 48 Def / 208 SDef
This ferrothorn can set up entry hazards, do damage, and heal. He is the total package. A great opener as well.
Relaxed nature (+Def, -Spd)