First,go to the front of Pokemon league(the tower)dong go in theres a cliff on the right if you have an AR cheat for walk trough anything walk of the cliff and you end up on route 224 and walk until you find a grey rock and walk 6 steps to the right you will see a lane going up with shaymin on it
feel free to catch him but the win standing there is fake you must go over the fake shaymin andwalk straight up you end up in a bigger space and you will see shaymin
North of the Pokemon league (Take the seabreak path)
Get Oak's letter and run across Seabreak path until you are in Flower Paradise and talk to Shaymin battle and Catch it if you like.
i think you need an action replay but i don't have one soo...
When you come to route 224 (Proffeser Oak wants you to in Oak's letter) you can thank anyone you want. It doesn't matter. After that, shaymin appears, and you chase her down seabreak path. Make sure your Pokemon isn't a high level, though. Shaymin is level 30. Hope this helped! ( P.S: use your bike on seabreak path, it's LONG!)
The best way to hatch Pokemon eggs is to cycle from solaceon town to twinleaf town and back .Or cycle up and down seabreak path
you must get oaks letter then go to route 224 where you will meet oak. he will tell you about the large rock on route 224 then ask you to put who your thankful for. suddenly seabreak path will appear.
The island to the right is where you can catch Shaymin. The one in the southeast is Seabreak Path. The one in the east is Flower Paradise, where you can catch Shaymin.
Press L and R to get the Oak's Letter, which activates the Shaymin event in Seabreak Path. 94000130 FCFF0000 B2101D40 00000000 0000B5D4 00000009 D2000000 00000000
You need the cheat "Walk through walls" to pass over the water. Then you are on the Seabreak Path, you have to ride till the end, there you find Shaymin and you can battle against him.
You know in victory road there's that one passage that's blocked by the old guy? Well after you've completed the national pokedex he moves away and you go up the path that happens to be in the sea. You find Arceus at the end of it.
if you mean a secret island with a legendary Pokemon on it, then yes , there is. there is new moon island, where you catch darkrai, and there's flower paradise on seabreak path where you catch shaymin. if you mean any other island, then no, there isn't. you can get these islands by action replay
get the mystery gift and go to any pokemart you will see a green guy talk to him and then go to the top of route 214 talk to professor oak then go through seabreak path. hope this helped :)