Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
So was there some form of change/kind of rule to balanced hackmons that I'm not aware of that would cause arceus dark to not be weak to fleur cannon? it had magic bounce and rocky helmet and to my knowledge there shouldnt be any reason it wasnt super effective.