Avoid his attacks with the cape and spin attack with the sword.
Same as the last phase, but hit din away from Onox with the Rod of Seasons when she's too close to him. If you have extra health, you can score double hits on Onox by just spin attacking through Din. Each spin attack through Din takes 1 heart, so make sure you have enough health to make it through the final phase.
When Onox slams down a hand to crush you, jump on it, charge a spin attack, and jump across to his other hand while releasing the spin attack. You should hit the gem on his forehead and land on the other hand so you can repeat the process. You can also jump on his hand when he does the swipe attack by jumping right as he finishes the swipe.