You need to beat the champion first.
Then you go to route 214 and you will
see a new entrance there and go in.
You also need to see all 151 pokemon
to get it
/\like pb10 said./\
once you do that go in a little far and go right. you will see a spot to use rock climb.
cynthia would be there if the requirements are not met.
run through the cave doors and you will find pillars. interact with 3 different pillars that have numbers less than thirty. if you get one with more than 30 you must restart.
once you do so giratina will be standing there.
(note: if you go through the door you came from you must restart.)
(another note: a rare bone will be in giratinas place when he is gone.