All right so I played around with the Evs, and so far the best spread I came up with is, timid 252 HP / 124 Speed / 36 Spdef / 96 def The idea of the ev's is somewhat simple, Max HP with 36 SpDef enables it to now take Specs Draco Meteors and recover the damage off. The 124 evs + a speed boosting nature enables Arceus to outspeed all Garchomp with around 3-4 extra speed for speed creeping those Lugia's that aim for that Garchomp benchmark. The rest is thrown into defense to severly weaken Extremkiller's Shadow Claw making only LO versions 2HKO. This enables Arceus to switch in on it when it tries to swords dance or revenge our genetic mutation and permanently cripple that Arceus with WoW (assuming it hits) or phaze it out. It also OHKOes Giratina with Judgment after 2 Stealth Rock switch-in's and takes around 31% from its shadow sneak. I also recomend we go for Roar over Perish Song since its only noteworthy mention is OHKOing those things that try setting up at the last moment and Deoxys-a already does a great job at punishing those boosters. Yay for forcing pokemon to do way more work then they need to :D, and checking all of Deo's major threats thats not scarfed Kyogre!