You have to follow the three legendaries (Mesprit, Uxie, and Azelf). They will show you little holes where you have to throw boulders in. Once you do that, you search for Cynthia. You and Cynthia will then find Giratina where you can catch it or make it faint.
If you accidentally make Giratina faint, you can always catch him again later...
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You to the place where you find giratina in diamond and pearl after you beat the game and go in the cave where you came out when you caught it, get past the 3 pillars in less than 30 rooms and there should be a portal to go back to the distortion world after the 3rd pillar and its at the end of the distortion world PS: this is how I got mine but look on youtube on how to get the Grisceous Orb on Platinum
if your where the three Pokemon uxie azelf and mesprit are in a triangle you go up one floor and then you push all three boulders down an roll them in the the hole that the three legendaries are and then talk to Cynthia then like a minute later youl be fighting giratina
if its platinum i don't think you can because it leads to the reverse world where you just came from and ther is nothing there to see or catch because there aren't any Pokemon in the reverse world its only a specific guess because im past that part that's wrong. In platinum after you defeat the E4 and Cynthia you can go back into the Distortion World as Cynthia is not blocking the Path anymore. There, you can find the Griseous Orb.
The Pokemon Lapras a water type Pokemon is the 131 Pokemon on platinum and can be found on Victory Road after getting past the guy standing in front of the doorway inside the cave.
you need the national dex