Not sure I'd class perfect evasion as Pokemon invincibility, personally. (read: I don't but your interp is valid)
And... gut says Alert spam after triggering Graviton Beam is enough? It's not great, but it should be able to beat him down just barely. I'll math it out later (it deserves that), but gut says that combo's good enough.
EDIT: Graviton Beam doesn't matter, Photon Rifle if you allow it works fine.
Bullet has 13 shots of Alert and is 100% accurate to average with Photon Rifle. It has 8 shots. (2437*8)/5497 is 3.55x PCHP, which should be enough to take down Giratina even with my high durability respect...
EDIT 2 NINJA: Wait, Giratina -does- get something I'd see bypassing Alert - Aura Sphere. ("Perfect evasion" loses out to an attack that literally cannot miss. Strike is just 100% accuracy to me, before you ask.)
Give me a minute, then.
EDIT 3: AHAHAHA DISREGARD THE ABOVE FAULTY MATH IS FAULTY, Bullet -needs- Graviton Cannon to stand a chance but he can't outpace Aura Sphere which 5HKOs him (EDIT: At worst for Giratina. More likely it 4HKOs). Giratina wins. Damn that's funny, thanks NEB for making me actually think it through.