AnswerYou have to install the "Run Anywhere" cheat in Action Replay; then go to the smooth, polished stone on the route to the right of the Pokemon League. Head north, and ignore the first Shaymin you see. Keep going North, through the extensively long Seabreak Path. When you reach Flower Paradise, "talk" to the Shaymin, and you will battle it.
Note: It is a level thirty.I have tested this; it works on both Pokemon Diamond and Pearl.
Shaymin Event Cheat
Works for both Diamond and Pearl
Press L +R to activate.