You need around 4 Geodudes with Sturdy. Teach the first one Toxic.
You start the battle with Geodude with Toxic. You use it. If you land it, then you just proceed to spam any attacks. No need to switch or anything. Spam attacks, let him faint you, switch onto another Sturdy Geodude. Spam attacks and switch fainted pokemon until Giratina faints.
If you miss Toxic, log out and try again. Good luck!