It depends on the plate its holding.
Arceus' type depends on the weaknesses of the plate's type it's holding. As a normal Pokemon with no plate, it's only weakness is fight.
Arceus can also only be one type at a type, due to only be able to hold one plate at a time, so there are no weaknesses/ resistances granted by dual types. Check the type chart to see what types are weak to what types.
EDIT: I misread/understood the Q; Arceus ??? type has no weaknesses, but cannot be used w/o hacking.