If you mean Richu, you have to evolve it. If you mean the legonday pokemon, use a MASTER BALL!
you need to catch and purify every shadow Pokemon then defeat the 100 battles on Mt. Battle
No it isn't possible through normal gameplay in any game of the series unless you count the Snag Machine in Pokemon XD and Colluseum.
You don't.
use a thunder stone on pikachu.
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You catch Ho-oh in Colluseum and you trade it to the GBA game, and then you migrate.
beat other Colosseum
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No you can't use Pokemon colluseum action replay codes on Pokemon xd
I don't know anywhere you can technically 'catch' and umbreon. You can catch an Eevee in the game though to evolve into an Umbreon from the Trophy Garden (for Pearl/Diamond/Platinum I believe you can either trade/migrate from Pokemon XD: Gale of Darkness, Pokemon Colluseum, FireRed, LeafGreen, Ruby, Sapphire, or Emerald or with someone else who has one in diamond, pearl, ect. as well)
you can only get it two ways, you can either cheat or transfer from Pokemon colluseum.
it is exactly like Pokemon colluseum except it's on the wii and you can do Nintendo wi-fi and battle with your ds.