Coverage:
To me this is kinda weird because Arceus can learn all sorts of attacks no matter it's types.
Resistance:
Steel Type:
Used because of all those resistances and immunity to Toxic. Very hard to defeat and like most Arceus can be a pain to defeat after using a couple of Swords-dances. Arceus can carry many stalling moves like Recover, Cosmic-power, Toxic, Wil-o-wisp, and so much more. Definitely hard to counter.
Source: PokemonDB