Ferrothorn @ Leftovers  
Ability: Iron Barbs  
Level: 50  
EVs: 252 HP / 140 Def / 116 SpD  
Relaxed Nature  
IVs: 0 Spe  
- Gyro Ball  
- Bullet Seed  
- Protect  
- Leech Seed  
Leftovers spread: Living hjk from mega lopunny, lele psychic nonspecs 4hko. Generally prefers protect due to leftovers but not neccessary.

Ferrothorn @ Choice Band  
Ability: Iron Barbs  
Level: 50  
EVs: 236 HP / 252 Atk / 20 Def  
Brave Nature  
IVs: 0 Spe  
- Gyro Ball  
- Power Whip  
- Bullet Seed  
- Knock Off/Explosion/Bulldoze
Choice Band Spread: Adjustment allows better living +2 ghost-z from mimikyu

Ferrothorn @ Iapapa Berry  
Ability: Iron Barbs  
Level: 50  
EVs: 252 HP / 100 Def / 156 SpD  
Sassy Nature  
IVs: 0 Spe  
- Gyro Ball  
- Leech Seed  
- Bullet Seed 
- Curse/Thunder Wave/Knock Off/Protect 
Spdef 50% berry: More spdef spread for lele, lives guaranteed 5 moonblasts from lele, lives fightinium-z from lele, lives most rolls of max spa modest specs lele hp fire.

Ferrothorn @ Iapapa Berry  
Ability: Iron Barbs  
Level: 50  
EVs: 252 HP / 28 Atk / 124 Def / 100 SpD  
Relaxed Nature  
IVs: 0 Spe  
- Gyro Ball  
- Leech Seed  
- Bullet Seed  
- Curse/Thunder Wave/Knock Off/Protect
Standard 50% berry: Defence for guarantee except 1 roll +2 ghostium-z mimi activates bulky berry, 4hko by breloom mach, 4hko by specs lele psychic, rest in attack

Ferrothorn has more move options shown here, generally curse, twave, knock off, protect can be interchanged in the last slot
A great Reference for Ferrothorn Spreads: https://www.ponz-poke.com/entry/2018/03/23/105738