sonic is best bird
Seed Flares coming off of its relatively high Special Attack makes Shaymin's Sky Forme a dangerous nuke, especially with a boosting ability. Not only does Seed Flare have ridiculous power, but its high chance of dropping Special Defense can turns 3HKOs into 2HKOs. It also has a blazing high Speed, outspeeding many offensive threats like Gengar and Latios. However, despite it's high nuking power, Seed Flare is more often than not resisted. While Air Slash seems to solve this issue at first glance, it lacks the immense fire power of Seed Flare. Shaymin-S also has an unfortunate weakness to Stealth Rock, meaning it becomes fairy easy to wear down. It also struggles with Flying-type Pokemon and Ice-type moves, especially the common Refrigerate abusers like Entei.
name: All-Out Offense
move 1: Seed Flare
move 2: Air Slash
move 3: Earth Power
move 4: Healing Wish / Psychic
item: Life Orb / Choice Specs
ability: Tinted Lens / Sheer Force / Serene Grace
nature: Modest / Timid
evs: 4 Def / 252 SpA / 252 Spe
Seed Flare is the obvious choice for its main STAB move due to its high BP - even resists that lack special bulk like Doublade struggle to tank it. Air Slash is your secondary STAB, and it helps mitigate Seed Flare's poor offensive typing. Earth Power is Shaymin's best coverage move, hitting the Steel- and Fire-types and pairing up nicely with both Air Slash and Seed Flare in terms of coverage. Healing Wish is a great utility move to use on the last slot, allowing a fellow teammate sweep once Shaymin weakens the enemy team. Psychic might not seem great at first, but it has good neutral coverage on the Sheer Force set. Primarily, it's your strongest hit against Delta Stream Zapdos.
Life Orb is probably the best item to run on Shaymin, due to how much power and flexibility it provides. It also pairs well with Sheer Force, negating the recoil if you use a move that is boosted by Sheer Force. Choice Specs, meanwhile, pair well with Tinted Lens, as this Seed Flare much more spammable due to Tinted Lens breaking through resists and Seed Flare keeping it's nasty side effect. Modest lets Shaymin speed tie with Thundurus-Incarnate. While this might not seem impressive at first, it means that Shaymin will outspeed base 110 and below such as Gengar and Mega Metagross. It also gives Shaymin the most fire power. However, Timid is still a very good option, as it lets you outpace much faster threats like Greninja. Sheer Force boosts the power of its STAB and coverage moves and nullifies Life Orb recoil at the cost of removing secondary effects. Tinted Lens lets it bypass would-be checks and counters like Doublade and Latias and allows Seed Flare to have its amazing side effect. Serene Grace is still insane with 60% flinch Air Slashes and Seed Flare almost always lowering Special Defense.
Shaymin-S is best used as a way to break the opponent's team, whether its offense or stall. It is also really good at cleaning up weakened teams due to its high speed. Because of this reason, it can serves as a win condition, depending on the match-up. Against offensive teams, Shaymin will find itself revenge killing the opponent's Pokemon, especially with Timid because of its high speed. Despite its role as a breaker and a cleaner, Shaymin-S can also switch into things thanks to its decent 100/75/75 bulk. It's Grass-typing can let it come Water-types that lack Ice Beam, although this should only be done if you don't have a better switch-in due to most Water-types using Scald. While Shaymin isn't a physical attacker, the burn will wear it down faster than it already gets worn down. It can also switch into many Grass-types, and force them out with the threat of an Air Slash. While Shaymin makes for a great revenge killer, you must be wary of switching into Stealth Rock often. You must also look out for potential Gale Wing and Refrigerate ExtremeSpeed users, as these things will destroy Shaymin. When Shaymin is low on health, Healing Wish is useful to switch into an injured teammate who can clean up.
Gale Wing and Refrigerate checks such as Skarmory, Entei, Heatran, and Doublade are great partners to it. Hazard control is also very important for Shaymin - Defoggers such as Latios and Spinners such as Excadrill for hazard control, while hazard setters like Heatran and Landorus-Therian are very helpful in wearing down the opponents, especially Flying- and Fire-types. Teammates like Mega Metagross and Heatran appreciate Shaymins ability to destroy bulky Water-types that bothers them. Shaymin finds itself at home with Volt Switch and U-Turn abusers like Scizor and Rotom-W due to Shaymin's ability to abuse the momentum granted from these moves. Magnet Pull users like Heatran make for great parters as well, removing pesky Steel-types like Skarmory.
move 1: Substitute
move 2: Leech Seed
move 3: Air Slash
move 4: Seed Flare / Earth Power
item: Leftovers / Toxic Orb
ability: Serene Grace / Poison Heal
evs: 120 HP / 252 SpA / 136 Spe
The combination of Substitute and Leech Seed is used to take advantage of Shaymin's great speed and decent bulk, along with ok defensive typing to wear down opponents. Air Slash is the primary STAB here to strike Grass-types that are immune to Leech Seed. It also lets you flinch opponents, especially when combined with Air Slash. Flinching with this set is even more devastating than with the other set, as a flinch usually means a free turn of passive healing thanks to Leech Seed. Seed Flare is a powerful STAB that lets Shaymin hit bulky Water-types hard. It can also potentially gain momentum due to its high chance to drop Special Defense. However, Earth Power has the best 2 move coverage with Air Slash, hitting Fire- and Steel-types that would otherwise effortlessly take Air Slashes. Protect is a usable option with Poison Heal, as it allows Shaymin to not only scout for moves, but also get free turns to heal with Leech Seed and Poison Heal.
The listed EVs give Shaymin's bulk a boost while allowing it to maintain high speed and power. 136 Speed with a Timid Nature lets Shaymin outspeed Thundurus-Incarnate and everything below it, allowing it to set-up Substitutes before the opponent can move. 252 Special Attack let Shaymin hit as hard as possible with Air Slash and Seed Flare. The rest is dumped into HP to enhance its bulk. An alternative spread of 4 Def / 252 SpA / 252 Spe can be used with either Modest or Timid for a more offensively-oriented spread. Modest with 252 Speed allows you to outspeed Thundurus-Incarnate while granting a huge amount of power, while Timid lets you outspeed very quick things like Greninja. Serene Grace is the primary choice to flinch things and get more health from Leech Seeds, while forcing switches with Seed Flare's insanely high chance to drop Special Attack. Poison Heal with Toxic Orb is also very effective, as it allows Shaymin to heal much more than it would normally, while letting it switch into Scalds much more comfortably due to the status immunity. It also provides an immunity to Paralysis, something that otherwise hampers Shaymin's potential.
You want to use offensive pressure to force switches in order to set-up Substitutes easier. The primary goal of the set is to wear down the opponent with Leech Seed and Air Slashes, allowing a teammate to sweep. While not primarily a revenge killer, Shaymin finds itself naturally able to fit in this role due to its high speed and reasonable power. Due to its good bulk, Shaymin can switch into resists much easier, such as Water- and Fighting-type moves. This is especially true with Poison Heal, as you can sponge Scalds much more comfortably due to the overall extra recovery and immunity to burn. Unless you have a Substitute, it is not very wise to try and challenge potential Gale Wing and Refridgerate ExtremeSpeed users, as they will destroy Shaymin even with bulkier EVs.
An answer to Gale Wings is an absolute must. Bulky Electric-types like Zapdos fit well for this job, as they can use Volt Switch to pivot into Shaymin to apply more pressure. Something that can answer Refrigerate is also very useful. Heatran is a great example of this. Hazard control is always appreciated, especially on this set since it wants to switch into things more often. Defoggers like Latias or spinners like Starmie make excellent partners, while Stealth Rock make Shaymin's job of wearing down the opposition much easier. Pokemon like Heatran or Skarmory fit the bill nicely. Finally, if you are not running Seed Flare, you might want to bring an additional answer to Water-types, especially those that can set-up or don't mind Leech Seed to much.
No Guard Grass Whistle is potent due to Shaymin's high speed, and makes it so that Air Slash and Seed Flare do not miss, but it doesn't offer much help to Shaymin offensively or defensively like its other abilities do. Adaptability also grants you huge power on your STABs while letting them keep their deadly secondary effects, but it's mostly outclassed by Sheer Force due to the latter boosting coverage moves and gives Shaymin-S more staying power due to not being worn down by Life Orb. Gale Wings can be used to surprise would-be checks with priority Air Slashes, but it also the power that Sheer Force does while not accomplishing much else. It is also redundant with Shaymin's high speed, and you still get outprioritized by ExtremeSpeed. Delta Stream is another option on the SubSeed set, as it mitigates its 4x weaknesses. However, it still has its fair share of weaknesses due to its Grass-typing, and its largely outclassed by Poison Heal in terms of switching into Water-types. While not dying to Ice Beam is nice, not getting burned by Scald is overall better - not to mention Ice Beam is going to do a lot regardless of Delta Stream.
Checks and Counters
**Gale Wings**: Gale Wings is the biggest problem to Shaymin, as they can destroy it even with its decent bulk due to its Grass-typing. Also, since most are Flying-type, this means that they resist Seed Flare, meaning it has to use the much less powerful Air Slash. It can kind of deal with the problem by running Gale Wings itself, but its still not in Shaymin's favor. Notable Gale Wing users include Braviary and Skarmory, whom can set-up on non-Tinted Lens Shaymin with Bulk Up and Swords Dance respectively
**ExtremeSpeed Users**: ExtremeSpeed is very dangerous for Shaymin, especially from Refrigerate Pokemon due to its 4x weakness to Ice. The most dangerous of these is Entei, as it also resists Seed Flare. Other forms of ExtremeSpeed are still dangerous, as Shaymin has no way to outpriorize it.
**Bulky Flying-types**: Bulky Flying-types are a huge chore for Shaymin, especially those who resist Flying as well such as Zapdos and Skarmory, as they are also immune to Shaymin's best coverage option. These Pokemon can usually switch in without much fear - even HP Ice isn't that big of an issue for them because of Delta Stream.
**Fire-types**: Fire-types also resist Seed Flare and can destroy Shaymin-S with powerful STAB options. Entei especially is threatening because it can run FridgeSpeed.
**Steel-Types**: Steel-types are an issue, especially if they can take an Earth Power through immunity or sheer bulk and neutrality. Keep in mind even things such as Levitate Doublade aren't big fans of ability boosted Seed Flares due to its sheer power.
**Choice Scarf**: Choice Scarf users are one of the few things that can outspeed Shaymin, especially Choice Scarfers that carry ways to OHKO Shaymin, such as Genesect.
**Special Walls**: Special walls such as Chansey are always a problem to special attackers, although Shaymin-S can muscle through them with some luck they are still an issue to deal with. A notable variant of special walls are Regenerator Assault Vest users such as Goodra or Meloetta, who can continuously switch in relatively healthy every time.
**Sap Sipper**: Sap Sipper Pokemon like Swampert and Gastrodon can take Seed Flares and Leech Seeds easily due to the immunity and potentially hit back with powerful Ice-type moves. These threats can be worn down with Air Slash, however.