+2 252+ Atk Life Orb Arceus-Normal Shadow Claw vs. 0 HP / 0 Def Giratina-O: 411-486 (93.19 - 110.2%) -- 62.5% chance to OHKO + Stealth Rock.
Also Arceus learns Shadow Force, no reason to not use it. Shadow Force (+0) 43.75% chance to OHKO after Stealth Rock