HandBananaNO

You can change Arceus's type to ??? using Pokemon Data Editor DS, but the sprite will still appear as normal. If we could combine the type with the sprite we could be on to something.

Awesome indeed. I think what may be the problem is that although it has the "???-type" sprite, it's still technically Normal-type (or at least that's how it turned out when I tried the codes on my game). If you can find a way to change it's base type to "???" that would probably stop it from reverting to Normal in battle. That said, the only ways I can think of to do that would be either trial and error with Pokesav's hex editor, or to hack in a ???-type Plate using the others as templates. Either way it would take a crapload of time.