An Umbreon UD can lock up any non-X'd Arceus, and running something with enough power to OHKO an Arceus without much harm to itself in return can also hurt it.
But yes, for already-stated reasons, DGX is the best Arceus counter. Just watch out for Bright Flame. (Gengar LV.X can help a tiny bit, but it involves a stage 2 line, so don't use it unless you're already running Gengar or a Psychic deck.)