It seems no one has post a fully stall set for this. I would like to post this:
BulkySubSeeder@Leftover
252HP/40Def/216SpD Impish ( Physical Defensive)
252HP/4Def/252SpD Careful ( Special Defensive)
Substitute
Leech Seed
Spikes
Protect
Ferrothorn looks like a very bad seeder at first glance as it is slower than nearly everything you'll meet. But it is astonishing good at this: as it forces a lot of switch by just stand there. It is the only SubSeeder resist Dragon that make it so hard to beat.
Under rain even Jolly Choice Scarf Chandelure's Flamethrower, one of the strongest attack Ferrothorn may meet, can never OHKO Ferrothorn even for the Physical defensive version . It is not surprising that Starmie can do no more than 20% without HP Fire. It can Substitute in front of a lot of pokemons.
When forcing a switch use Substitute. Seed the one come in while he attack your Sub; Protect to gain a free turn of damage and recover. If oppents switch in something cannot damage you, set up Spikes.
By doing this, you suffer a resisted hit ( mostly no more than 25% ), gain 3 leftover recovery and 2 leech seed recovery, recover approximately 45% health, nearly as Recover while make your oppenent lose 1/4 of their health. This set work excellently with entry hazards. With 3 layer of spikes or stealth rock, this indeed deals nearly 1/2 to oppents while you recover 1/2 of your health. It is so devasting to many All-out attacker especially those without recovery. Also, consider Fire Blast has only 8PP, it is very easy to stall it out by Substitute+Protect one by one in 4 turns, especially for Choiced attacker ( Leftover is needed to perform this effectively)
It seems very easily be setup fodder, but it still deals with them in emergency. For example, when at full health, SD LO Garchomp will never OHKO Physical defensive Ferrothorn with Earthquake. It make Garchomp suffer 4 turns of damage even if Ferrothorn doesn't under Substitute at first. SD/Seed, EQ/Protect, EQ/Spikes, EQ/Protect. 4 Seed + 1 Iron Barb + 1 LO = 72.5% health. And after the hit Ferrothorn recover 2 Leftover + 2 Seed ~ 25% health (Consider Garchomp has 0HP EV). Then you have 50% chance KO Garchomp ( the chance of Protect) with 1 layer of Spikes+ Stealth Rock(12.5%+6.25%) + 1 leech seed(12.5%) = 103.75%. If you don't wanna rely on 50% protect, you can set up 3 Spikes+Stealth Rock which make Garchomp fainted on 4th turn, or Toxic Spikes+Stealth Rock( 1 layer is enough to kill Garchomp). Some Garchomp will predict and SD twice on the 2nd turn while you have only 50% protect on 3nd turn to avoid being OHKOed, while you can predict this and set up Substitute, make it have no hope to break Ferrothorn by any means.
As a slow seeder Ferrothorn boasts something other fast seeder cannot: Seed the one that bring in after U-turn or Volt Change. Slow U-turn can break Substitute while bring-in one without being seeded. But there's no slow U-turn for Ferrothorn at all. Even 0 Spe EV paralyzed Scizor is faster than Ferrothorn. What makes U-turn even worse is that most U-turn cannot break its Substitute - U-turn from Scarf Flygon deals only measly 20% to Physical Defensive Ferrothorn. 0 Atk Scarf Geneksto without download Atk deals 23-27%. There's no commonly seen Volt Change can break the Substitute, except CT from Offensive Zapdos.
Team options:
As mentioned above, this set must be used with entry hazards. But it is better to use in a balanced or semi-stall team (I found it hard to use in a full stall team, as Ferrothorn is a good attacker in such a team and better use Gyro Ball, Power whip etc. and it give too many same Weakness as Forretress and other Steel types, which are useful to remove hazards, setup other entry hazard and provide resistance to Bug-, Ice-, Fly- attacks.) A Ghost is necessary to abuse entry hazards. Stealth Rock and Toxic Spikes is appreciated. Considering Fire Blast stall is very common, prepare a Wish passer to restore its health after a lot of Substitute is good.
This set's legendery counter is Liquid Ooze Tentacruel, make Ferrothorn lost 1/8 health when seeded and spin away your spikes. Prepare a Ghost to block rapid spin and a strong Electric /Psyhic/Ground attacker to remove it is good. It is a pity Rotom-A lost Ghost type makes countering Tentacruel a little harder.
Taunt can shut this Ferrothorn coldly, so an attacker is needed to abuse the fact opponent may Taunt instead of attack.
This Ferrothorn easily be forced out by Fire or Fighting attacker from which gain one free hit or setup. So your team should prepare resistance or immunity to them. Flash Fire Chandelure can come in on Fire/Fighting with no damage (Shadow Tag one can function well, too). Bulky water, especially Jellicent is also good at it. Moltres and Charizard can also come but hindered by Stealth Rock. Dragonite, Salamence and Gyarados are traditional switch-in and works well. Tentacruel(Defensive) and Starmie(Offensive) are good parters as they provide rapid spin while resistance to Fighting- and Fire- attacks. Ditto and Unware Quagsire are also good as they can deal with other's set up.
Defensive set up with recovery is the worst thing this Ferrothorn may meet, as they can set up without problem and recover Leech Seed damage. Magic Guard users, like Sigilyph and Reuniclus, can do anything they wanna do in front of it. So pair Tyranitar, Scizor or Escavalier is good idea though the latter to they extended Fire weakness.
With leftover it is deemed to death when faced Chandelure. So pair a strong Pursuiter like Tyranitar to eliminate Chandelure is also appreciated. But Leftover largely increase the power of this set.
I used this set in both OU and Uber enviroment and successfully bring down a lot of Choice Fire Blast and Overheat from Dialga, Palkia, Heatran, Ninetales and, mostly, FB/OH Chandelure, or set a lot of Spikes, which help my Fire-weak sweepers like SD Scizor, SD Breloom, LS Serperior and CM Bronzong to sweep.
