Ferroseed can only be caught within Chargestone Cave as a relatively common encounter on all floors.
Behold, GameFreak's attempt at making the ultimate mixed tank. And... they succeeded. Boasting the incredible Grass/Steel-typing and a competent attack stat to back up its immense mixed defensive capabilities, the Ferroseed Line is a powerful addition to your team, and will serve you well in your adventures in Unova. Just keep it away from anything with a Fire-type move.
N (Chargestone Cave): Not a terrible start for the line all things considered. While his own Ferroseed is best avoided for the sake of convenience, everything else is manageable through what it knows naturally and its TM list. Boldore and Klink lack ways to do more than really weak neutral damage while you can hit them super effectively with Metal Claw and Bulldoze respectively. The same applies to Joltik, however you can only hit it neutrally with Bulldoze.
Gym #6 - Skyla (Mistralton City, Flying-type): More things that Ferroseed can wall, and it only gets bulkier with the eviolite. Apply ample servings of Leech Seed and let the passive damage destroy... or you know, send in an Electric-type and get the job done faster?
Cheren (Route 7): If it is not named Simisear or Pignite, feel free to place the annoying little spike ball of doom down and watch him struggle to break it. Gyro Ball and Curse is an additional method which can be deployed against his Liepard, Unfezant and Grass-types. The Fire-types are to be avoided on the pain of death however.
Gym #7 - Brycen (Icirrus City, Ice-type): Nothing here can hit either stage of the line super effectively. Just set up curses on Vanillish and spam Gyro Ball.
Bianca (Route 8): Once more, if it is not a Fire-type, Ferrothorn laughs at it and beats it up between Gyro Ball, Power Whip and Leech Seed. Just don't engage Emboar/Simisear, please.
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type): The only thing that comes remotely close to threatening Ferrothorn is Druddigon's Revenge and the fact it is A. Almost as Slow as Ferrothorn, B. has Rough Skin in Black and C. Revenge is a 2HKO at full power. However if you decide to stall it out, Revenge suddenly becomes a 4HKO, significantly less threatening against Leech Seed, the brutal Iron Barbs/rocky helmet combo and a hyper potion. Fraxure dies trying to set up with Dragon Dance, while Haxorus needs to be at plus 6 to 2HKO with Dragon Tail, you really should not let that happen.
Cheren (Route 10): See Bianca for the elemental monkeys and starters. Liepard and Unfezant still fail to do anything of note.
Elite Four Shauntal (Pokémon League, Ghost-type): The only thing Cofagrigus can do to Ferrothorn is burn it. While this may be annoying, Ferrothorn is well suited to engage in a stall war. Fire off a Toxic and a Leech Seed and prepare for a long one. If you have Curse, you can also set up on it. Jellicent is laughable, and cannot hit you neutrally, use Power Whip a few times and be done with it. Golurk is in the same boat as its Brick Break is only a 3HKO at best while Power Whip is a certain OHKO. Chandelure is the last thing Ferrothorn will see if it stays in for some reason. Fire Blast will kill your Pokémon.
Elite Four Marshal (Pokémon League, Fighting-type): Be careful in this fight, as Ferrothorn is at a type-disadvantage. However, there are things that it can get away with here, depending on what it knows coming into this fight. For example while Mienshao's High Jump Kick is a 2HKO at equal levels, protect is a free way to get half of its health gone, leaving it open for picking off next turn between your STAB move of choice, preferably Gyro Ball, and Iron Barbs. Sawk is generally doable as Karate Chop has a chance to 3HKO, while Power Whip is a certain 2HKO. While Throh is more threatening since both Storm Throw and Power Whip are sure 3HKOs with it being faster, potion support should let Ferrothorn come out on top, particularly when it should be losing a quarter of its health everytime it hits Ferrothorn. Sadly, this where Ferrothorn's wild ride ends for this fight, as Conkeldurr is not happy with you beating up his mates with a Steel-type, cleanly 2HKOing with Hammer Arm, which is something Ferrothorn does not want to deal with.
Elite Four Grimsley (Pokémon League, Dark-type): Liepard is a joke, it will do Fake Out on the first turn, so popping an X-attack or defense might be nice, but it will do more to itself through recoil. Power Whip dispatches it effortlessly. Krookodile's Earthquake is a 4HKO, meanwhile if you deployed that X-attack, Power Whip will OHKO regardless of intimidate (2HKO's if Ferrothorn has its attack lowered without the boost). Bisharp can only hit you neutrally through Aerial Ace, which is a 7HKO, Bulldoze is a 2HKO with the recoil of Iron Barbs and the rocky helmet but then so is Power Whip. Scrafty has Brick Break, however it is only a 3HKO and Power Whip promptly OHKOs back after the recoil damage Ferrothorn throws back.
Elite Four Caitlin (Pokémon League, Psychic-type): Reuniclus finally stops Ferrothorn's rampage as Focus Blast is a 2HKO and it is faster. However speed creeping is an option here and you only need to have 53 speed to do it (do note this is 100evs on a neutral nature, 68 on a positive one at level 50) and Power Whip 2HKOs as well. Everything else? Cannot hit Ferrothorn for more than neutral damage outside of Sigilyph's Ice Beam and Air Slash and those are 5-6HKOs on max HP Ferrothorn (which is not difficult to get due to Audino grinding).
Reshiram/Zekrom (N's Castle): Sending Ferrothorn out against Reshiram is the definition of suicide. However it does mess with Zekrom quite nicely, making it a good choice for weakening it if you intend to save the masterball for later.
N (N's Castle): See the above points for his own dragon. Carracosta is allergic to Power Whip, and Sturdy will have N waste his full restores. Vanilluxe is outdamaged as Gyro Ball 2HKOs in contrast to Blizzard's 3HKO. It is a similar story for Archeops, only Gyro Ball OHKOs it. Klinklang requires Bulldoze to take down, which has a high chance at a 3HKO, however Klinklang barely scratches Ferrothorn, even after Metal Sound. The main problem here (Outside of Reshiram in White), is Zoroark, which has Flamethrower to melt Ferrothorn with. It can also disguise itself as his other Pokémon, so, for example if Klinklang randomly comes in on Ferrothorn and it is not his last Pokémon, expect Zoroark and bail.
Ghetsis (N's Castle): Cofagrigus is a Toxic-Protect staller, Ferrothorn is a Steel-type which resists its attacking moves, do the maths. Bouffalant and Bisharp both have issues damaging Ferrothorn to boot, which allows it to pick them off. Seismitoad is literally down in one Power Whip but this is where Ferrothorn's fun ends. Both Eelektross and Hydreigon pack Fire-type coverage in the forms of Flamethrower and Fire Blast respectively. Avoid at all costs.
Post-Game: Continue walling everything that does not have a Fire or Fighting-type move of significant power.
When first caught, Ferroseed will come armed with Metal Claw, Pin Missile, Gyro Ball and Iron Defense. These provide a relatively strong start for the line, particularly Gyro Ball which hits hard given the line's extremely low speed stat. At level 30, Ferroseed attempts to spice things up with Mirror Shot however, The line is a mixed tank with competent physical attacking abilities, this is not the right move for it. Next up at level 35 is Leech Seed. Let's take a moment to admire just how useful this move is here, granting Ferroseed/thorn a reliable source of recovery and chip damage. The last move that Ferroseed learns before it starts trying to evolve is at level 38, Self-destruct, should be obvious why this is utterly terrible in a Nuzlocke. It does learn things after this, but Ferrothorn also gets them.
As a Ferrothorn, the line starts off by trying to learn Power Whip upon evolution. It's hard to say no to a STAB move with base 120 power and compensates for Ferrothorn's average attack stat. Iron Head at level 46 is generally outclassed by Gyro Ball due to the line's slowness, which also removes the odds of it ever making an foe flinch. Payback at level 53 is likely the last move that the iron fruit will pick up in the main game, and does work well with its STABs and stats, however it is relatively useless in the one or two fights you'll have it by. In the Post game, Flash Cannon has the same problems as Mirror Shot and Explosion is as bad as Self-Destruct.
From the move relearner, Ferrothorn does not get much of note, Rock Climb is outclassed by Return should you chose to run Normal-type coverage on it and Roll Out is not that good a move. However Curse is an amusing option if the Ferrothorn is to take a more offensive route. Through TMs, the line gains a cluster of additional options, mainly on the offensive side. Toxic and Protect allow for Ferrothorn to become the ultimate in annoyers in conjunction with Leech Seed. Thunder Wave serves more as a team support option to deal with fast threats due to indirectly weakening Gyro Ball. In terms of attacking coverage, Ferrothorn has Shadow Claw and Payback for Ghost and Psychic-types, Aerial Ace to hit Grass, Bug and Fighting-types and finally Bulldoze to smack Steel and Electric-type Pokémon. As a final note, if breeding is allowed in your run, Ferroseed can get Stealth Rock from either a Roggenrola line or Dwebble line father.
Standard: Gyro Ball, Power Whip, Leech Seed, Toxic
Water-types: Ferrothorn's main issue is that problematic x4 weakness to Fire-type moves. Fortunately, while the options are somewhat limited in Unova, the Water Pokémon accessible provide a solid answer. As a bonus, Ferrothorn devours every Grass-type and most Electrics that threaten its supporters.
Ghost-types: While Ferrothorn is a spiky wall of pain for any physical attacker, it does not enjoy taking repeated hits from the strongest Fighting-types Unova has to offer. Therefore a Ghost-type helps by alleviating the pressures facing Ferrothorn in being the almighty physical wall as a result of their immunity. Meanwhile most Dark and Ghost-types looking to harm Ferrothorn's new friends find themselves on its spikes.
Jellicent: While this Pokémon as already been noted for meshing with Ferrothorn well, the sheer extent of their defensive synergy is too grand to ignore. Leaving only Ground and Flying-type moves unresisted between the two of them is only the tip of the many reasons why these two are perfect partners in a defensive backbone for a team.
Offensively focused Pokémon: While Ferrothorn's sheer bulk and decent offensive abilities allows it to handle most of the game in combination with Jellicent, the two appreciate having strong attackers to deal with the few threats that they cannot take on, such as Shauntal's Chandelure and Ghetsis' Eelektross.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Ferroseed Line Ratings
What Nature do I want? Anything that does not take away from its defenses or physical attack is ideal, i.e. Brave.
At what point in the game should I be evolved? If you are keeping up with the level curve, you should have a Ferrothorn around the time you fight Brycen, if not immediately afterwards.
How good is the Ferroseed line in a Nuzlocke? It is, for lack of a better word, broken. The moment it joins your party, this line immediately makes its presence felt. Should it be the line walling pretty much everything that game has to face it, before removing its foe with clinical precision. Even in the line's one bad type match-up, Marshall, it can still put in a lot of work.