* He's immune to all other attacks: Phantom Ganon is specifically designed to be vulnerable to only arrows. He's immune to swords, magic, bombs, and any other weapon you might have.
* He's a final boss: Both games have a limited number of arrows by design, and you're expected to use them strategically for the final battle.
Here's what you can do if you're low on arrows:
* Try to conserve your arrows: This means avoiding unnecessary hits on him, and focusing on hitting his weak points (his head and chest) for maximum damage.
* Use the Big Goron: In *Ocarina of Time*, if you have the Big Goron's help, he can distract Phantom Ganon while you shoot arrows at him. This can help you conserve your arrows.
* Explore your surroundings: Both games have arrow pickups in the boss fight. You might be able to find some while you're fighting Phantom Ganon.
Ultimately, though, if you run out of arrows, you'll need to find a way to replenish your supply or you won't be able to defeat Phantom Ganon.