The only 'quite usable' set, which I found for this Arceus is Swords Dance (yeah, I know, Ghost and Normal are totally superior, but... I see no other options, Calm Mind is totally outclassed by MewTwo). I posted some calculations in 'Project Arceus Reservation' thread. Set, which uses STAB Zen Headbutt, and surprisingly the only thing, which you can't 2OHKO or OHKO as neutral targets are Giratina, Lugia and Max HP/Max Def Groudon. Even Supporting Groudon can die in 2OHKO 75% of time with only SR up. Just some food for thought ;). Still nothing great, but I guess better then nothing.