The Multitype ability would have to be upgraded to reduce the super effective damage of its type while still maintaining the ability to use the plates; sort of like Delta Stream's effect but more broken since it's just one type.
However, even with this it cannot be as broken as the ability known as Desolate Lands (kinda a cheat way to get rid of that 4x water weakness Groudon :/ ) - let's also nullify crits as well.
Since Arceus has 120 base in all stats, make it so that the Mega gives a 120 increase instead of the peasant's 100 and allow for Arceus's HP to increase proportional to its percentage health.
Now you have a nice 840 bst Arceus which defies the HP rule of Mega evolutions and virtually has no weakness - Might as well make it so that the ability cannot* be copied, nullified, or bypassed to get rid of peasant tracers and scrub Mold Breakers.
Also, ditto dies if it dares to try and transform into M-Arceus as we have no need for a false god.
Slap on a Steel Plate and enjoy the glory of so many resistances, no weaknesses, and an immunity.
Little chance of getting lucky against this God and no, you cannot try and toxic stall this.
I would try to make it broken but the plates kind of limit what I can think of.