Also, I think you missed a rather important move for Giratina: Spite.
PP Killer Giratina.
Giratina (Another Form) @ Leftovers
Ability: Pressure
Nature: Impish
252 HP/248 Defense/ 12 SpD
~ Substitute
~ Spite
~ Roar
~ Dragon Claw
A rework of the anti-Arceus set, but the idea is fairly simple, set up a Sub to absorb their attack, then Spite away their PP. You could alternatively use Protect, but Sub stops Blissey from doing anything productive to you. Roar to phaze things out, and Dragon Claw so that Taunt doesn't ruin you.
With so many Low PP moves, Spite Giratina can set up a foe to be unable to counter your opponents pokemon. Having to spend 2 PP just to get around Giratina's Sub is bad enough, but when it can Spite away 2-5 more PP and then switch out, it gets ugly pretty fast, even in a realm of Maxed PP.