This thread's been asleep for a painfully long time. I theorymonned this Giratina while contemplating what to do with the Modest one in my Pearl from my casual days. Keep in mind that my Ubers experience is very limited.
Giratina (A) @ Leftovers
Modest (which may not be the best at all)
252 HP/124 Def/124 SpD/8 Spe
Will o' Wisp
It's a fake dual-booster in that any physical attacker who attempts to break it risks a burn, and Pain Split is the best recovery for a non-Resttalk set. This Giratina is intended to disrupt the opponent's play by increasing its appropriate defense and forcing the opponent to switch frequently, building hazard damage and subjecting them to burns and Dragon Pulses.
REGIGIGAS is so CUTE.
My wifi occasionally acts up, but that's the only reason you'll see me disconnect. ;)