When Togekiss first showed how brutal Serene Grace could be in Diamond and Pearl, we all breathed a slight sigh of relief that Togekiss was not that naturally fast of a Pokémon, thus requiring setup to get to the truly tortuous Flinch Hax conditions. Then Platinum came and one upped Togekiss with Shaymin's Sky Forme for a whole new level of disgusting Flinch Hax. Base 127 Speed is still an impressive benchmark even after 4 generations of power creep, and one upping Togekiss' Nasty Plot is Seed Flare, whose guaranteed Metal Sound rider thanks to Serene Grace made Shaymin-Sky nigh unwallable. However it still has its problems. Seed Flare requires sticking around several turns to get the most out of it, which is hard for Shaymin-Sky thanks to its horrid Defensive typing. Additionally while Base 127 Speed is impressive, there are faster threats running around like Mewtwo and Deoxys that can easily force it out.
Shaymin -Sky Form
Ability: Serene Grace
Item Attached: Life Orb
EVs and Nature:
4 HP / 252 SATK / 252 SPE
Jolly (+SPE, -SATK)
Shaymin-Sky makes a great hit and run artist/revenge killer. Come in on something Shaymin Sky threatens out like Groudon, click Seed Flare to mess up whatever comes in and proceed accordingly. If Shaymin Sky is Faster, reaching for Air Slash spam can reach KOs otherwise not possible, while Earth Power punishes Dialga and Arceus Steel. Healing Wish rounds out the set. If Shaymin Sky is about to be KO'd have it go out on its terms and bring a teammate back up to fighting shape.
Choice Scarf -Enables Shaymin Sky to outspeed the entire unboosted game.
Choice Specs -gives Shaymin an appreciable power boost but at the cost of flexibility.
Metronome (the item) -good for Air Slash Spam
Dazzling Gleam -hits Rayquaza, Giratina and Arceus Dragon
Land Form Shaymin has always lived in the shadow of its Sky Form. Base 100 Stats across the board have long ago stopped being impressive with only the base 100 HP being truly above average. Mono Grass is horrible offensively and defensively, giving a myriad of Pokémon that resist Shaymin's STAB and plenty that threaten it in turn. However, Seed Flare is still a house of a move as 70% is still a generous proc chance for the Metal Sound rider. Additionally Shaymin has decent coverage getting both Fairy and Ground (requiring transfer) to cover the Dragon, Steel and Fire types that resist its STAB. Support wise, while Shaymin doesn't have the abundance of support fellow mythical Celebi does, access to Aromatherapy and Healing Wish is fantastic with Natural Cure making it easy for Shaymin to save Aromatherapy PP. Shaymin can fill a variety of roles both offensive and supportive, as long as one manages its Grass typing.
Shaymin -Land Form
Ability: Nature Cure
Item Attached: Leftovers
EVs and Nature:
252 HP / 4 DEF / 252 SATK
Shaymin provides great role compression with both offense and defense with this set. Seed Flare's laurels have been sung repeatedly, and makes for a great move to apply pressure -Blissey is nowhere near as capable as sponge with its Special Defense halved. Aromatherapy provides great utility for the team. Cure Scald Burns, Toxic Poisoning, Paralysis and get your team back into fighting shape. Rest gives Shaymin incredible recovery, its downside mitigated by Natural Cure wiping away the sleep. Dazzling Gleam rounds out the set, giving Shaymin some teeth against the many, many Dragons that would otherwise pivot into Seed Flare without issue.