I've read before that there is a simple way to get a Lv. 100 Shaymin in Pokemon Platinum. I'm not exactly sure if it works, because I don't have Pokemon Platinum, but the way the anonymous guy online wrote it, it was kind of convincing. So here it is.
1. Pick Chimchar as your starter.
2. Give your Chimchar, Monferno, or Infernape a Potion to hold.
3. Get the Odd Keystone. The easiest place to get it would be in Twinleaf town. To get it, Surf down in the water, then use the Poketch app Dowsing Machine to find the Odd Keystone hidden in the trees.
4. Put the Odd Keystone in the Hallowed Tower, an item that looks like a crumbling well south of Solaceon Town.
5. Go to the Sinnoh Underground and talk to exactly 50 people, instead of 32. 32 is for Spiritomb, one of only 2 Pokemon with a type pairing that, under normal circumstances, has no weaknesses. The other is Sableye, a Hoenn Pokemon.
6. Go back to the Hallowed Tower. Try to make sure you have a Master Ball, and save in front of the figure that has appeared in front of the Hallowed Tower. Also, if you want a specific nature for the Shaymin, have an open space in your party so that you won't have to catch it and go all the way back to a town to look at it.
7. Talk to the figure that appeared by the tower, and use your Master Ball on it. If you can't get a Master Ball, Get a Scyther at the highest level you can, and make sure it knows False Swipe. Also, have a huge stock of Ultra Balls, Timer Balls, 4 Quick Balls, and Dusk Balls. Only get Dusk Balls if you want to catch Shaymin at night.
Using this glitch, I think that you can get Shaymin. I read this glitch on http://answers.yahoo.com/question/index?qid=20090409064341AApjF8p.
