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Arceus is the capital G God of Pokemon and isn't even competitively legal anyway, so just make it unapologetically broken. Forget the plate bullshit.
Multitype has Arceus change type every time it uses a move. Like gen 6-8 Protean.
If Arceus uses Judgment, it always works like Legend Plate, to be as effective as possible.
And defensively, Arceus always changes to be as resistant to incoming moves as possible. Immune if applicable, 4x resistant if not.