Beat the game, and return to Sendoff Spring. Cynthia will be gone, and you can enter the cave she was previously blocking. Enter Turnback Cave, and go to the end to find an item and the entrance to the Distortion World. Enter it to find the Griseous Orb. Give it to Giratina, and it can stay in its original form outside Distortion World. You can now catch Giratina in Turnback Cave.