The best strategy for defeating the boss of Arbiter’s Grounds is by using the spinner. Spin along the side of the sand pit he’s in the center of until you’re directly behind him. Hit his spine with the spinner’s attack to damage him. Each time, more skeletons will come up to block you from reaching his spine. Keep killing them so you make a gap where you can spin through to hit the spine again. After hitting Stallord a few times, the sand will drain and the head of Stallord will remain. Use the spinner to raise the platform and then also use the spinner to climb the platform. He’ll eventually start shooting fireballs at you while you’re climbing up, which can only be avoided by jumping from track to track on the other wall. Stallord will then get beside you, giving you a chance to jump and attack his head with the spinner. When on the ground, attack the sword that Zant put in Stallord’s head to eventually kill him.
During the second stage, after you break Stallord’s spine and he comes back as a floating head, there’s a good trick. When you’ve knocked him down, spin attack the Twilight Sword on his head non-stop. Don’t charge the spin attack. Do about one or two regular sword slashes and then spin attack over and over. If done right, you can kill him without having to knock him down again.