After you recieve the Mystery Gift at GameStop or through Nintendo WFC wherever you are it will be automatically saved on your game file before you pick up the Pokemon at the Pokemon Center. Just make sure you don't save the game after you get the Mystery Gift Pokemon from the Delivery Man cause If you do you're stuck with whatever Nature and IV's that Pokemon had. Natures and IV's are 99.9% of the time given out randomly except for Level, Movesets, and any held Items it might have.
That's why after you get the Mystery Gift, upload your game file, and get the Mystery Gift Pokemon from the Delivery Man don't save the game, keep checking the Pokemon's Summary to make sure it has the right Nature and IV's you want and If it does save the game. Even If you don't get the right Nature and IV's on the Pokemon and you soft-reset to where you last saved the Delivery Man will still be there at the Pokemon Center to give you your Pokemon. I'm sure alot of people know about this but it does help for those who don't know.
This method is called soft-reseting and it usually comes in very handy for getting specific Natures and IV's for Event Pokemon, Breeding Eggs, or in Gen IV getting the Berry you want from the Berry Master. Unfortunately Berries are now limited to the Dream World in Gen V in which the best way to get them now is by Wi-Fi Trade or from someone who already has them from the Dream World. I guess If they used Action Replay to get them it could work but I wouldn't recommend it though.
That's why after you get the Mystery Gift, upload your game file, and get the Mystery Gift Pokemon from the Delivery Man don't save the game, keep checking the Pokemon's Summary to make sure it has the right Nature and IV's you want and If it does save the game. Even If you don't get the right Nature and IV's on the Pokemon and you soft-reset to where you last saved the Delivery Man will still be there at the Pokemon Center to give you your Pokemon. I'm sure alot of people know about this but it does help for those who don't know.
This method is called soft-reseting and it usually comes in very handy for getting specific Natures and IV's for Event Pokemon, Breeding Eggs, or in Gen IV getting the Berry you want from the Berry Master. Unfortunately Berries are now limited to the Dream World in Gen V in which the best way to get them now is by Wi-Fi Trade or from someone who already has them from the Dream World. I guess If they used Action Replay to get them it could work but I wouldn't recommend it though.