The only way that you can get oak's letter is by visiting a nintendo event.one comes to england once per year.Go to that event to get shamin then get it to max happiness then go to the town next to the windworks around that town is a blond girl talk to her and she will give you a flower (key item)use it on shamin and it will transform into sky shamin.Then you will have the legendary sky shamin.Sky shamin willtransform back to shamin at night so in the morning you willneed to give it the flower again....