I would like to put Shaymin (both forms) in the StratDex.
SeedHax (Sky Form)
Skymin@Leftovers
Serene Grace//Modest/Timid
EV/ 252 HP/ 236 Speed/ 20 Spatk (Timid)
EV/ 252 HP/ 244 Spd/ 12 Spatk (Modest)
-Substitute
-Leech Seed
-Air Slash
-
Secret Power/other move
Not quite like the Parahax from Togekiss or Dunsparce, but it's the fastest Subseeder in the game, and has Air Slash to keep fellow Grass Pokemon away. Also has access to 101 Subs to Subseed Blissey and Hopefully heal enough for another Sub when it manages to take yours down. Secret Power is there because in WiFi Battles, it inflicts Paralysis, and with a 60% chance of doing so, who wouldn't? So it gets free healing with the flinch, paralysis, and if they manage to attack, they'll only have a chance at breaking your Sub, which then you get to put another one up. Those EVs are depended on what nature. Modest with the EVs provided set it at 351, outspeeing +Speed nature, 252 EV Pokemon at 110 base speed(Gengar and co.) by one point, assuming you have max IVs in Speed and HP to provide the 101 Subs. The Timid set it at a speed of 384, outspeeding the likes of Weavile and Swellow running +Speed nature and 252 EVs by one point. You can alter the speed to whatever you like, just dump the rest into Spatk. And if you don't like the paraflinch thing, you can use Seed Flare/Earth Power/HP Fight or Ice. This is off the top of my head
Specs/Scarfmin (Sky Forme)
Skymin@Choice Scarf/Specs
Serene Grace//Modest/Timid
EV/ 252 Spatk/ 252 Spd/ 4 HP
-Seed Flare/Energy Ball
-Air Slash
-Earth Power
-HP (Fire/Ice)
Simple Specs/Scarf set. Seed Flare for power and a chance to lower Spdef, or Energy Ball for more PP and accuracy. Air Slash for coverage, same for Earth Power. And HP Fire to get what the three other moves can't: Levitate Bronzong, Skarmory, and Zapdos. Or HP Ice if you don't like dragons. Leaf Storm isn't shown because Seed Flare can be used. So why go for a little more power with a big Spatk drop, when you can get your unique move that works like a Grass type Fire Blast and can lower the opponents' Spdef?
Support (Land Forme)
Shaymin@Leftovers
Natural Cure//Bold
EV/ 252 HP/ 252 Def/ 6 Speed
-Seed Flare/Energy Ball/Grass Knot
-Aromatheraphy
-Rest/Leech Seed/Synthesis
-Grasswhistle/HP (Fire/Ice)/Leech Seed/Earth Power
In my opinion, this is the moveset that Land Form does better than the Sky Form, but is usually compared to Celebi. But there's Seed Flare, Aromatheraphy, and Grasswhistle that Celebi doesn't. Seed Flare is what you should choose, but there's Energy Ball for PP and accuracy, and Grass Knot for 120 power on heavier foes. Aromatheraphy to rid your team of status. Choose your form of healing in the forms of Rest, where you switch out afterwards to cure the sleep, then Leech Seed if you want to force switches and heal somewhat (use with Spike/SR support), or Synthesis if you like that. Then choose HP Fire or Ice like the other movesets, Earth Power for Fire Pokemon, or Leech Seed if you want it that bad.
Subseed (Land Form)
Shaymin@Leftovers
Natural Cure//Timid
EV/ 252 HP/ 252 Speed/ 4 Spatk
-Substitute
-Leech Seed
-Seed Flare
-HP Fire/Ice/Earth Power/Air Slash
It's the other form trying the Subseed now. It has better defenses, so the Subs are a little harder to break. Seed Flare to make some more switches and more damage, and the HP of your choice. Fire to kill incoming Scizor and other Steels not named Heatran, Earth Power for Heatran, Tyranitar, and others weak to it (watch out for Flyers!) or HP Ice for other Grasses and dragons.
Extra Options
You could try a physical set with the sky form, but it should be mixed. Just slap a Naive/Hasty nature for Speed or Lonely/Naughty for Attack, Life Orb, 252 Att and 252 Speed EVs, Seed Bomb, Zen Headbutt, Return, Swords Dance, Seed Flare, or Natural Gift if you need something taken care of. But it does special attacks better, so it's novelty and very suprising. Other than that, Celebi is better most of the time, and if you use Shaymin, it's going to be the Sky Form because it's just better. Another thing, this Pokemon tends to cause switching, so please have Spikes and/or Stealth Rock support. Maybe a little Toxic Spikes as well.
EV Explanations
-The SeedHax EV spreads are explained, but there are other milestones you can drop to. 228 EVs in Speed and Modest get you 347, which is one above Infernape's max Speed. Then you can run 216 EVs in Speed and Modest to go past Scarf users with base Speed of 55 with max Speed. But you really shouldn't. The HP is explained in the SeedHax set and applies for the Land Subseed.
-Support should run max HP and Def, because other Pokemon take Special hits better.
-The Choice set should run max Spatk and Speed unless if you are insane.
Troublesome Pokemon
If you don't have Earth Power, Heatran walks all over you. Without HP Ice, Dragonite and Salamence and Skymin think of you like a walking salad. Not having HP Fire leaves you vulnerable to Skarmory and Scizor, and walled by Bronzong. When you're running Specs and Scarf, look out for Cresselia and Blissey, they may have Ice Beam. If there's no Sub up for Skymin, Weavile and Mamoswine have Ice Shard to take you down. And U-turners will be a nuisance to the Subseeding sets.