The Distortion World is a unique, gravity-defying dimension. Follow Cynthia through this strange world. You'll need to solve environmental puzzles by moving rocks and navigating platforms. This area is linear but visually disorienting. You'll eventually reach Giratina. This is your chance to catch the legendary Pokémon! Giratina is Level 47. Save before battling it, and use status effects and strong attacks to weaken it. If you accidentally knock it out, it will reappear after you defeat the Elite Four.
After dealing with Giratina, you'll exit the Distortion World and find yourself back on the Sendoff Spring. Head east to Route 214 and then south to Valor Lakefront. You can now access the Spring Path, which leads to Turnback Cave, where you can re-encounter Giratina if you failed to catch it earlier.