Here's the deal with shiny Pokémon:
* Shiny Pokémon are random: The chances of encountering a shiny are very low, typically 1/4096 in most games.
* There's no way to manipulate it: You can't make a Pokémon shiny by using certain items, tricks, or waiting a specific amount of time.
* It's all luck: It's completely random whether you'll find a shiny or not.
So, if you're hoping to find a shiny Giratina, your best bet is to keep searching, be patient, and hope for the best!