Giratina-Origin @ Griseous Orb
Ability: Levitate
EVs: 252 Atk / 128 SpD / 128 Def
Adamant Nature
- Shadow Force / Shadow Claw
- Outrage / Dragon Claw
- Earthquake
- Stone Edge
Explanation: Shadow Force/Shadow Claw is for STAB, Shadow Force has higher power but takes two turns, though it can penetrate Protect/Detect (it's like Dig/Dive/Bounce) while Shadow Claw deals damage with increased critical hit ratio. I prefer the former, but it's up to you. Outrage/Dragon Claw is for STAB and again, Outrage has higher power but causes confusion while Dragon Claw is just... there. I prefer the former again but it's up to you. Earthquake provides coverage (against Poison, Rock, Steel, Fire and Electric types) with high power so it's good to have. Stone Edge also covers several types, namely Flying, Bug, Fire and Ice, the major weakness of Giratina) with an increased critical hit ratio.
Alternatively:
Giratina (Origin) @ Griseous Orb
Ability: Levitate
EVs: 252 SAtk/ 128 SDef/ 128 Def
Modest Nature
- Dragon Pulse
- Shadow Ball
- Earth Power
- Aura Sphere
Explanation: Dragon Pulse and Shadow Ball are basically the Special STAB versions of the Physical Outrage/Dragon Claw and Shadow Force/Claw duo in this case. Earth Power is the Special version of Earthquake and again provides coverage against Poison, Rock, Steel, Fire and Electric types. Aura Sphere provides coverage and never misses and is notably strong against Normal, Rock, Steel, Dark and... Ice, Giratina's major weakness.
Extra: Hex + Thunder Wave/Will-O-Wisp may work quite well since Hex's power will double to 130. I haven't tried this before, so it's just a theory.