Generally, giratina-o are on the offensive side, but here's a sample set i just made if you want to test it out:
Giratina-Origin @ Griseous Orb
Ability: Levitate
Level: 50
EVs: 236 HP / 4 Atk / 28 Def / 4 SpD / 236 Spe
Adamant Nature
- Shadow Force
- Icy Wind / Tailwind / Shadow Sneak
- Will-O-Wisp
- Protect
- 252 SpA Fairy Aura Xerneas Moonblast vs. 236 HP / 4 SpD Giratina-O: 216-254 (84.7 - 99.6%) -- guaranteed 2HKO
- Outspeeds Smeargle by 1 point
- 252+ SpA Primal Groudon Eruption (150 BP) vs. 236 HP / 4 SpD Giratina-O in Harsh Sunshine: 87-103 (34.1 - 40.3%) -- guaranteed 3HKO
- 252+ Atk Primal Groudon Precipice Blades vs. 236 HP / 28 Def Giratina-O: 103-123 (40.3 - 48.2%) -- guaranteed 3HKO (Under Gravity)
- 252 SpA Primal Kyogre Water Spout (150 BP) vs. 236 HP / 4 SpD Giratina-O in Heavy Rain: 91-108 (35.6 - 42.3%) -- guaranteed 3HKO
However,
- 4 SpA Life Orb Mega Rayquaza Draco Meteor vs. 236 HP / 4 SpD Giratina-O: 320-377 (125.4 - 147.8%) -- guaranteed OHKO
- 4 SpA Life Orb Mega Rayquaza Draco Meteor vs. 252 HP / 252+ SpD Giratina-O: 229-273 (89.1 - 106.2%) -- 37.5% chance to OHKO
Not great vs rayquaza/xerneas in any case :/
EDIT: this is a sample top cut team that uses giratina-o if you're looking for inspiration:
3rd Place Taiwan - Hung-Wei Lai:
common partners include:
Kyogre, Kangaskhan, Sableye, Gengar, Smeargle, Thundurus, Groudon, Aegislash, Mawile and Liepard
That's a lot of ghosts..