You cant cuz there hasn't been a mystery gift for it yet. the only way would be to cheat
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What is the che cheat to get an arceus in Pokemon diamond? What is the che cheat to get an arceus in Pokemon diamond? What is the che cheat to get an arceus in Pokemon diamond?
You can catch Arceus in Diamond, Pearl, or Platinum.
Pokemon Diamond, Pearl, and Platinum have Arceus.
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arceus