Use Azerim's part under one of the lower blue crystals to hover up onto the catwalk. At each end you'll find an elevator platform. As soon as you step on it, a spike will shoot out of the hole near the platform and the platform lowers to the ground.
Use this to hurt Quetzalcoatl several times. You should stand up there ready for stepping the platform. As soon as Q is covering the hole do a side step and let the spike hurt it bad.
I don't remember how often you must hit Q this way. Good luck!