You don't legendaries don't breed except for manaphy and phione. Anyone saying different is either purposly giving wrong info or doesnt know what theyre talking about. Check sites like serebii or bulbapedia for proof.
Try breeding your Giratina with a Ditto, that should do it, as ditto can copy any pokemon. Also, I believe if you breed Arceus with Ditto, the result will be an egg of either Palkia, Dialga, or Giratina
Plus I heard that you can breed a heatran with a ditto and darkrai.
I tested breeding giratina and ditto it does not work.I beat the elite four and I am a experienced trainer.I believe arceus can not breed and other legendery pokemon except rotom and manafy I am not sure with heatren.