You need to get Arceus. Which is a whole other story.
You need to get arceus via event in pokemon diamond/pearl/platinum and transfer arceus to hg/ss you have to then go to the ruins of alph under violet city and go to the first house you see. For this event to occur I think you may need to finish every stone puzzle first. I'm not sure. The when you try to go in, the research leader will rush out the door and bump into you. He will take you to the area where you get dialga/palkia/giratina. To get arceus you must use the azure flute in spear pillar on diamond/pearl/platinum. You get the azure flute thru promotional event. You cannot hack to get arceus but you can hack to get the flute if you hack to get arceus, the ds will know and you won't be able to get giratina. For it to work you need to use the azure flute to get arceus.
You need to trade arceus to heart gold or soulsilver the go to the ruins of alph and start to go in the house a man will walk out and talk to you. After that go in the nearest cave and go down the ladder.Then then another man will talk to you so listen to what everyone says.You will figure out the rest.(Arceus can't hold any plate or it wont work.)
You need to get arceus event then go to some city
My friend traded giratina for palkia!my palkia was lv.87 but my friend gave me lv.100 giratina!