Oh yeah, Turnback Cave is a pain. I'm not too sure if this si true, but supposedly you must head in the door that is closest to the rock formation that is different from the rest. For example, if all of the rocks from an x but one side is different than the rest, enter the door on that side. Once in a while you'll hit a pillar. the top number, I believe, shows the rooms you already went through. If you go through 30 and still don't make it to the end, you'll be forced to "turnback", or go to the entrance.
If you hit all three pillars, the next room will lead you to...
He has a few Ghost and Ground Attacks, so a Normal/Flying will take no damage from all of Giratina's attacks except from Slash.
After defeating/catching him go back to Turnback Cave. Complete the puzzle in a certain amount of rooms to get a prize where Giratina was originally standing. Prizes include Stardust and a Reaper Cloth, along with another which I forget.