Well I see not many people have tried it but here is how this goes down.
Getting Shaymin hacked/cheated way; TWTWT(The walk through wall trick)
Steps
1, Activate code. (Walk through walls)
2, Reach the route blocked off by the man who won't let you in without a national dex.
3, go to the dead end and save
4, walk to the cut off path and hop on your bike. (This area allows it because it's not a unknown zone this area is part of the game, but is cut off due to the fact it's an item event(see bottom for event way))
5, Drive on until you reach the doll/figure/sprite/ect.
6, resave.
7, Go up to the pokemon and press A and thus begin battle
8, use a poke/great/ultra/master/ect. Ball on the Shaymin until caught.
9, You have your Shaymin
10, Gloat about your Shaymin you caught using the only logical way you can if you can't get to a store that is holding the shaymin event.
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Catching Shaymin the (semi-)TRUE Way.
1, Wait for the event(Too late for that though)
1xa, (This counts for cheating) Acess the action replay and activate the code for oaks letter.
2, Goto a Pokemart and obtain the letter from the gentleman in green by the counter.
3, Once again head to the man blocking the route and show him your national dex and head to the dead end.
4, watch cut scene. (WHAT!!! It's not really a cutscene but whatever.)
5, Begin chase with Shaymin(ride bike like before.
6, Reach end of the trail and save your game.
7, Walk up to Shaymin and press the A button thus beginning battle.
8, Same as before fight it use a Ball of any type on it.
9, If you caught it congrats you now have bragging rights that you caught Shaymin the (semi-)Legit way.
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Hope most of you have the idea now.
-Your welcome