Okay, first of all, i haven't tested this cause Arceus hasn't come out yet, so don't get mad. What You Need: a 2 DS Pokemon Games (Pokemon Diamond,Pearl,Platinum), a second DS, a mystery gift. (if you don't want to restart your game, disregard this cheat)The Arceus movie will come out next year, so this cheat will activate then. When the 12th Movie Event Arceus comes out, use the mystery gift WiFi(because if you go to Toys 'R Us 17 times, the might get suspicious ;) ) and get the Arceus and Picu(shiny) then trade the Arceus over to your Pokemon Platinum, Diamond, or Pearl. Then restart your game you got the Arceus from. Play the game until you are able to get the mystery gift. Repeat. Keep on doing this until satisfied with amount. Don't worry, you have enough time. this will run on for about 2-3 months(I know because serebii.com posted when it started and when it ends, and the Japanese wouldn't get a longer time then the U.S. and Europe but if they do, I'll post an apology). This can work for any DS game that has a Pokemon giveaway.
obviously this is my best cheat ever if it works =D
P.S. If your a fan of Shiny Pokemon but can't find any like me, I THINK it'll work the same way with the Shiny Pichu. What makes it easier if you want both, I THINK, repeat, THINK (emphasize the THINK) that they are given away at the same time, since they are both 12th Movie Event Pokemon.