To get Giratina you will need to beat the game and have the Upgraded Pokedex.

Go to Vielstone City and go down.

Go Right and then go south. You will see a path that is IMPOSSIBALE to miss. Go and take to path.
You will need Rock Climb.
You will also ENcounter LV 50s.
After going up the thing go around this path and then go down again using rock climb and there should be a cave inside you will need to do a puzzle.
After you worked it out you will find Giratina.
ANd there is also storng pokemon inside the gave.

Hoped I Helped You.