Forum Thread
Giratina
Forum-Index → Help → Answered → GiratinaDuring the minigame, the screen will fade in and out/moderately flash as Giratinas appear and disappear all over the screen. Your task is to click a specific number of them before your time runs out. This minigame repeats a couple of times with the number of Giratinas (and the time you're given) increasing each time.
After you clear all rounds of the minigame, you get a Spooky Manor Map that you can use to send a pokemon on a rumble mission to receive a Dark Orb. It's a summon item you can use at the Ancient Cave to get a Giratina egg.