The only real offensive counter to Giratina is Baxcalibur, due to its wisp immunity and 120BP STAB super-effective move and choice band off of 145 base attack being able to KO Giratina, but it cant even switch in to a draco meteor and Glaive Rush barely misses a KO even with that insane attack
252 Atk Choice Band Baxcalibur Glaive Rush vs. 252 HP / 252+ Def Giratina: 404-476 (80.1 - 94.4%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Giratina Draco Meteor vs. 0 HP / 4 SpD Baxcalibur: 318-374 (85.7 - 100.8%) -- guaranteed OHKO after Stealth Rock
Defensive counters barely exist, due to Tera Steel to avoid toxic. Giratina could also switch out, giving a free switch to the likes of

or

to beat the defensive mon.
Even Chi-Yu can't OHKO specially defensive Giratina with a specs set
252+ SpA Choice Specs Beads of Ruin Chi-Yu Dark Pulse vs. 252 HP / 252+ SpD Giratina: 374-444 (74.2 - 88%) -- guaranteed 2HKO after Leftovers recovery
After Giratina tanks a hit, it strikes back with either thunder wave, will-o-wisp, or just a Draco Meteor off of a respectable 100 special attack and your mon dies
tldr giratina does not belong in OU it is way too busted even for current power level
yes i double posted but this is just dumb