Moves
Seed Flare gives Shaymin a niche, as it is the strongest special Grass-type move without drawbacks. Grass Knot has issues hitting low weight Pokemon such as Clefable and Rotom-A, while Leaf Storm has harsh stat drops. Seed Flare OHKOes offensive Suicune after Stealth Rock and comes close to OHKOing Gyarados and Tyranitar after Stealth Rock. Seed Flare's 40% chance to drop Special Defense helps Shaymin power through Clefable, as Clefable is 2HKOed by Seed Flare if the first drops Special Defense. Hidden Power Ice is Shaymin's best option to hit Dragon-types, as it OHKOes Flygon and Dragonite after Stealth Rock. Shaymin has a chance to get through Latias if Latias switches into Seed Flare and gets the Special Defense drop, as that KOs offensive Latias after Stealth Rock and heavily pressures defensive Latias. Hidden Power Fire is another option, as it hits Steel-types such as Skarmory, Scizor, Metagross, and Jirachi hard. Both options at least 2HKO Breloom.
RestTalk is Shaymin's best option both for recovery and for helping it with Seed Flare's meager 8 PP. Both Seed Flare and Hidden Power threaten Breloom, the most prominent Sleep user in the tier. Shaymin is unique among Sleep absorbers in that it has Natural Cure, preventing it from being sidelined by Sleep either via Rest or opposing Sleep users until it wakes up after it switches out. That is a double-edge sword though, because it means every time Shaymin tries to absorb sleep, it risks being hit hard instead.
Set Details
Shaymin needs maximum Special Attack EVs and a Modest nature to be as threatening as possible, since it has poor coverage and relies on Seed Flare's power to handle common threats such as Tyranitar and Gyarados, which aren’t even OHKOed with maximum investment. Maximum Speed EVs gives Shaymin a solid Speed tier, outspeeding Rotom-A (with Hidden Power Ice, as the IV drop from Hidden Power Fire causes it to tie with Rotom-A), offensive Suicune, most defensive Latias, and neutral-natured Lucario. Shaymin has naturally good bulk, but it can also run HP EVs instead of Speed EVs to better survive attacks from Breloom, +1 Gyarados, and Suicune.
Usage Tips
Shaymin typically comes in on its resistances or on sleep moves to make progress, as Shaymin is so uncommon that its presence is unexpected and can either check what it needs to or absorb sleep in the short term. Shaymin typically comes into the game pretty early to blunt momentum from Water-types, Ground-types, and Breloom to save other defensive walls such as Clefable and Latias from having to take these Pokemon on. However, once Shaymin comes in, it needs to be extremely careful around Spikes users such as Skarmory, Forretress, and Roserade, especially if it doesn’t have Hidden Power Fire for the former two. Entry hazards—not only Spikes but especially Toxic Spikes—are a death knell for Shaymin; Shaymin both lacks reliable recovery and is grounded, so if it can’t get a Rest off, it’s effectively fainted. Try to use Rest before Shaymin is forced out, as it isn’t extraordinarily bulky and can be KOed rather easily by what it’s supposed to check if it’s heavily worn down.
Offensively, try to hide Shaymin’s coverage move as long as possible. Early in the game, it can be hard to tell if Shaymin has Hidden Power Ice or Hidden Power Fire, or even if it’s a more offensive set and potentially has Earth Power. Opponents might be hesitant to send in their Dragonite, Heatran, or Forretress in on Shaymin for fear of its coverage so try to take advantage of this. However, Shaymin doesn’t have a lot of Seed Flare PP and only can get more while asleep, so don’t use it too flippantly. Additionally, be careful against Breloom. Once Shaymin is revealed, every time Breloom comes in, it could just start attacking to get through Shaymin. This gives an opportunity to switch to a stronger, faster Breloom check like Latias to force out Breloom without getting put to sleep.
Team Options
Shaymin is most common alongside Magnezone or Choice Scarf Magneton support on more defensive teams for a few reasons. The foremost reason is that Shaymin enjoys Skarmory and other Steel-types out of the picture so it can use Hidden Power Ice with impunity. The second reason is that defensive teams with Magnezone often have large defensive holes, especially against Water-types, as they are using a frail Pokemon instead of another wall. Shaymin helps fill that hole, as it is an untrappable check to Water-types and Breloom. Shaymin’s goal against more offensive teams is to keep other walls such as Clefable and especially Latias healthy against strong Water-types such as Empoleon, Gyarados, Kingdra, and Swampert. As such, Clefable and Latias are fantastic teammates. Against more defensive teams, Shaymin is a solid wincon against them once Clefable is incapacitated. To pull this off, Shaymin appreciates teammates that can take a lot of defensive responsibilities such as Hippowdon, Wish + Protect Jirachi, and Swampert. Swampert and specially defensive Hippowdon are especially helpful, as they cover Shaymin’s weaknesses quite well. Shaymin really enjoys defensive Clefable sets that can beat other Clefable, such as one using Encore, Sing, or even Focus Punch. Even if it doesn’t, just having Knock Off is very helpful both to weaken opposing Clefable, but also to Knock Off Skarmory’s Shed Shell should Clefable get the chance early in the game. Additionally, Shaymin can support more offensive Latias sets such as Calm Mind + 2 attacks.
Shaymin can fit on more aggressive teams in a Fire-Water-Grass core, but these structures often really struggle to deal with Latias, hyper offense, and entry hazards and Shaymin faces competition from Breloom and Roserade. Common partners for this structure include Heatran for the Fire-type and a host of Water-types such as Swampert, Suicune, Kingdra, Starmie, and Gyarados. Other common partners include revenge killers such as Choice Scarf Flygon, Choice Scarf Rotom-A, and Choice Scarf Latias and strong offensive partners such as Swords Dance + Pursuit Scizor, Metagross, and Jirachi.