I'm not entirely sure this would be a good idea in competitive play; Origin Forme is more offensively geared based on its stats, and Altered Forme is better off with Pressure unless you're playing Doubles/Triples. Plus, it's in Ubers so you're less likely to meet Florges or any Ice type that isn't a Kyurem. I do like the idea behind Hex/Will-O-Wisp but that seems more likely for Altered Forme than Origin IMO.
Here's what I'd go with, personally:
Giratina-O @ Griseous Orb
Levitate
Modest/Timid
4 HP, 252 SpAtt, 252 Speed or 252 HP, 4 Att/SpAtt, 252 Speed
- Shadow Ball
- Thunderbolt/Draco Meteor
- Earth Power
- Aura Sphere
All-out SpAttacker. Levitate would actually be useful for taking on Groudon, switching in on PB and smacking its face in with either Draco Meteor or Earth Power, depending on if you go DM or TBolt. DM is obviously far more powerful but you won't get quite the same level of coverage. But then, in this tier, there's very few pokemon that can resist Dragon type anyway, and those that can are hit pretty hard by Earth Power.
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Now, on to my set:
Tyrantrum @ Life Orb
Rock Head
Jolly
252 Att, 4 SpDef, 252 Speed
- Hone Claws/Aerial Ace
- Dragon Dance/Rock Polish/Aerial Ace
- Head Smash
- Earthquake
Sort of a doubledancer I guess? I'm not sure if Rock Head's exactly gonna be the new hotness, and I'm also pretty sure it won't be enough to push Tyrantrum up into a higher tier, but it's still an interesting option that gives it a VERY powerful Head Smash, second only to Rampardos and maybe Reckless Emboar (who both take enough damage to nearly or completely kill themselves with it anyway). Head Smash pairs well with EQ due to Rock/Ground synergy, and anything that resists both is weak to Aerial Ace (with all but Torterra doubly weak to it). Of course you have your setup moves; DD is the classic, while you could use Rock Polish instead if you think its power is good enough without a boost. Hone Claws is a good replacement as well; while it doesn't make Tyrantrum any faster and is a dead giveaway to what kind of set you're running, it does still give you an offensive buff and makes it so your Head Smash is 100% accurate.
Of course this set isn't without its issues. It's highly susceptible to Special attackers and physical, rock-resistant walls, especially Torterra that could well eat an Aerial Ace easily and kill you off with a Wood Hammer. While Rock Head does keep you from killing yourself with Head Smash, Life Orb damage is still gonna stack up, and combined with lacking SpDef, middling HP, and a slew of weaknesses, he's not exactly going to be sticking around for the long haul. It's probably a good idea to use a specially defensive Fire type to resist its Ice, Fairy, and Steel weaknesses, since it's more capable of handling Ground and Fighting attacks which are typically physical. Even in that case, it's best not to rely on him to sweep until late-game when you've removed most of the things that'd wall him.