I tried using flash but it wouldnt let me
If you have the Key Item (also Event Item) Oak's Letter, Professor Oak will meet you at the rock when you talk to it. He will ask you a series of questions. Once answered, Shaymin appears. The entire route will become Flower Paradise and by going north, you can catch Shaymin.
The event has passed, so you can't do this without hacking.
Click here for more information and visuals.