That Arceus had Mold Breaker, not Multitype and therefore counted as Arceus-normal. I'm not sure whether it's still supposed to display as Arceus-ghost when it's like that, though (evidently he put it as Arceus-ghost in teambuilder).http://pokemonshowdown.com/replay/balancedhackmons-246161546
when I use a ghost attack on opponent's Arceus-ghost, it doesn't have any effect for some reason. this happened twice, once with shadow ball and again with judgment(I was holding spooky plate)