I didn't know we were posting VGC. Why not lead with this:
Giratina-O@Griseous Orb
Nature: Mild
Ability: Levitate
EVs: 252 Atk / 216 SpA / 40 Spe
-Draco Meteor
-Shadow Sneak
-Outrage/Dragon Claw
-Aqua Tail/other physical move/Earth Power/Aura Sphere
Metagross@Choice Scarf
Nature: Jolly
Ability: Clear Body
EV's: 252 Atk / 252 Spe / 4 HP
-Explosion
-Trick
-Earthquake
-Rock Slide/Bullet Punch/Meteor Mash/Iron Head/Elemental Punch
Explode if the enemy gets any funny ideas. Draco Meteor to murder what explosion can't kill. Shadow Sneak to laugh at sashes. Trick if you expect a trick room. EQ when Draco Meteor won't quite get the job done and explosion seems illogical. Rock Slide if they're in the air, or if you prefer, Meteor Mash for pure power or Iron Head for reliability. Bullet Punch only if you are one of those people that tricks a lot. Elemental Punches are purely for type coverage, but be warned that Giratina is weak against Ice and relies on its steel counterpart to take them out. For Giratina, you could opt for a Lonely nature, which is what Smogon recommends, but I say Mild so that Draco Meteor can nab some crucial 2HKOs. Can't think of moves for Giratina. Outrage is rather unstable in Double battles so you might want to go for Dragon Claw, but at the same time, Dragon Claw rarely gets that OHKO. Aqua Tail is because gira doesn't have a very diverse physical movepool, but it can prove to be useful. Just so you know how powerful Draco Meteor is, anything that doesn't resist it and isn't in Ubers is crippled and left to die from Shadow Sneak, if they didn't die from Draco Meteor first. There are probably a few exceptions, but it really is uber powerful, and here you are putting it on one of the highest SpA to get STAB on it, plus the 20% from Griseous Orb. If you manage to hit any of the non-resistant Dragon Types(i.e. Salamence, Latias, Latios, Garchomp, Rayquaza), then its an instant OHKO, no EV's or nature could save their life. For the last slot, you could go for WoW or T-wave. Destiny Bond is something I would use for the lol points, and is a more sophisticated way of using protect. Aura Sphere is to get rid of Ice types that Metagross can't kill. Earth Power is for the Physically Bulky types that resist Explosion, such as an enemy Metagross or Steelix(but who uses Steelix). If you have any suggestions(that's what I'm counting on) please share.
Partners: WishBliss is by far the best partner for Giratina-O. It replenishes his health and gives him another chance to Draco Meteor to oblivion. Not to mention their great synergy. Blissey sponges anything not super-effective, and what do you know, Giratina gets a clean entrance against those fighting attacks. On the offensive, you could choose another partner that can explode, just to take advantage of Giratina's ghost type, but seeing as how VGC allows another Uber, you might as well go extreme here. Mewtwo is a good Uber that you can use as well as Palkia, the latter of which cannot explode. They both have good coverage, but while Mewtwo is faster than Palkia and just a tad stronger, it is less bulky too. You could always go with other Ubers such as Dialga, but it is best to choose something that can hit fast, wide, and strong.
Damage Calcs.
Metagross Explosion vs. 0 HP / 0 Def Neutral Palkia = 145.5% - 171.3%
Metagross Explosion vs. 252 HP / 252 Def Positive Palkia = 78.6% - 92.7%
Metagross Explosion vs. 252 HP / 64 Def Neutral Lugia(Great Wall Set) = 76.2% - 89.9%(finished off with Shadow Sneak)
Metagross Explosion vs. 0 HP / 0 Def Neutral Salamence = 152.3% - 179.5%
Metagross Explosion vs. 252 HP / 252 Def Positive Salamence = 88.3% - 104.1%
Metagross Explosion vs. 4 HP / 0 Def Neutral Kyogre = 133.9% - 157.6%
Metagross Explosion vs. 252 HP / 0 Def Neutral Kyogre = 113.4% - 133.4%
Metagross Explosion vs. 240 HP / 244 Def Neutral Kyogre = 89.3% - 105.2%
Metagross Explosion vs. 4 HP / 0 Def Neutral Groudon = 91.5% - 107.9%
Metagross Explosion vs. 252 HP / 32 Def Neutral Groudon = 75.7% - 89.1%
Giratina-O Draco Meteor vs. 0 HP / 0 SpD Neutral Metagross = 50.5% - 59.5%
Giratina-O Draco Meteor vs. 252 HP / 0 SpD Neutral Lugia(Great Wall Set) = 45.7% - 54.1%
Giratina-O Draco Meteor vs. 4 HP / 0 Def Neutral Kyogre = 60.8% - 71.9%
Giratina-O Draco Meteor vs. 252 HP / 0 Def Neutral Kyogre = 51.5% - 60.9%
Giratina-O Draco Meteor vs. 4 HP / 0 Def Neutral Groudon = 88.9% - 104.7%
Giratina-O Draco Meteor vs. 252 HP / 32 Def Neutral Groudon = 70% - 82.7%