Overview
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Ferrothorn has solid defensive stats and a good defensive typing with lots of resistances, while also having solid defensive stats. It is commonly seen on teams as a strong check to Kyogre, Latios, and Latias. Ferrothorn's support movepool is very wide, consisting of moves such as Stealth Rock, Thunder Wave, Spikes, and Leech Seed. Its decent Attack stat combined with Gyro Ball allows it to check many speedy threats such as like Mega Gengar and Xerneas. However, Ferrothorn is complete Taunt bait, is very slow, and has a nasty 4x weakness to Fire-type moves.
Support
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name: Support
move 1: Spikes
move 2: Gyro Ball / Power Whip
move 3: Leech Seed
move 4: Protect / Thunder Wave / Toxic
ability: Iron Barbs
item: Leftovers
evs: 252 HP / 4 Atk / 252 SpD
nature: Sassy
Moves
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Ferrothorn is one of the best users of Spikes entry hazard setters in the Ubers tier due to its solid bulk. (I hope this change is okay? these first four sentences were all pretty short and I felt like we could combine some) It fares very well against the most common Defoggers, Latios and Latias, especially if it is running a status move such as Toxic. Spikes also allows Ferrothorn to pressure defensive teams. It is also a good user of Stealth Rock due to it's solid bulk. Gyro Ball is for faster Pokemon such as Mega Gengar and Xerneas, while Power Whip hits Kyogre, Arceus-Ground, and Palkia. Power Whip also packs quite a punch against neutral targets. Leech Seed helps Ferrothorn recover health, and combined with entry hazards, it can be used to chip away at the opponent's foe's health. Protect goes very well with Leech Seed and Leftovers, allowing it Ferrothorn to recover more HP, (remove comma) while also scouting against Choice-locked Pokemon. Thunder Wave can be used to cripple setup sweepers, but it should not be used with Gyro Ball because Thunder Wave will lower Gyro Ball's power. Toxic can wear down common switch-ins such as like support Arceus formes and Defog users, and therefore it has good synergy with Spikes.
Set Details
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Iron Barbs helps Ferrothorn wear down physical attackers such as like Mega Kangaskhan. Leftovers gives Ferrothorn passive recovery, helping it avoid some crucial 2HKOs and 3HKOs. 252 HP and Special Defense EVs allow Ferrothorn to better check lots of special attackers, such as like Kyogre, while a Sassy nature further boosts Ferrothorn's special bulk. 0 Speed IVs should be used with Gyro Ball, because it strengthens Gyro Ball to strengthen it. Investing in Defense is also not a bad idea, as it helps Ferrothorn check powerful physical attackers such as like Swords Dance Arceus variants Formes.
Usage Tips
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You should aim to use Ferrothorn's good defensive typing and bulk to your advantage by using it to set up entry hazards reliably. Ferrothorn should be taking hits and trying to wear down the opposing Pokemon with residual damage, such as from Leech Seed, entry hazards, or Toxic. Around common Fire Blast or Focus Blast users such as like Dialga, Palkia, and Mewtwo, use Protect to scout if they have it. Also keep in mind that Gyro Ball only has 8 PP, so use it only when you have to.
Team Options
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Ho-Oh is another good teammate for Ferrothorn, as the two have perfect synergy together, and Ho-Oh loves Spikes support to wear down its checks and counters. Lugia can come in on Fighting-types and a few Fire-types easily and phaze to take advantage of Spikes. Arceus-Water can check Ho-Oh, Defog, while also being a good check to Fire-types in general (do you mean Arceus is using Defog? then say "Arceus-Water can check Ho-Oh and other Fire-types in general as well as provide Defog support" or something). Setup sweepers appreciate Ferrothorn setting up entry hazards, (remove comma) and using Toxic and Leech Seed to wear down bulky walls that they cannot muscle past. Ferrothorn also loves cleric support from Pokemon like such as Blissey or Clefable, as it is easily worn down by burn damage.
Other Options
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You can opt to run Rocky Helmet over Leftovers, as this makes to heavily damage Pokemon that make contact with Ferrothorn take a lot of damage,; (semicolon) however, the passive recovery from Leftovers is usually better. Knock Off can be used to neuter some Pokemon such as like Latios or Latias holding a Soul Dew.
Checks & Counters
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**Fire-type Pokemon**: (colon) Fire-type Pokemon such as Mega Blaziken, Primal Groudon, and Ho-Oh can roast Ferrothorn with their respective STAB moves. However, both (you listed three, though) are neutered by Thunder Wave or and Toxic.
**Taunt**: (colon) Taunt users such as Yveltal completely shut Ferrothorn down, (remove comma) but must be wary of Gyro Ball, Leech Seed, and or status.
**Darkrai**: (colon) Darkrai can easily cripple Ferrothorn with Dark Void, (remove comma) and proceed to use it as setup fodder if running Nasty Plot.
**Mega Sableye**: (colon) Mega Sableye stops Ferrothorn from setting up entry hazards, (remove comma) and takes pitiful damage from its attacks.
**Powerful Physical Wallbreakers**: (colon) Powerful Physical wallbreakers like Primal Groudon can easily beat Ferrothorn with Fire Punch, and Rayquaza can easily break through this set with Dragon Ascent or V-create. Mega Salamence takes little damage from Gyro Ball, (remove comma) and doesn't care about status if it is running Refresh, while it can easily use Ferrothorn as setup fodder. However, all of them are annoyed by Leech Seed.
**Fire-type Coverage Moves**: (colon) - (remove dash) Ferrothorn must be very careful around Pokemon that commonly use Fire-type coverage, such as Fire Blast Dialga, Palkia, and Mewtwo as well as the occasional Hidden Power Fire from Mega Diancie and Xerneas.