This is the final boss battle of the game, against Ganon.
When you start off the battle, your Master Sword will be swatted away. Use your Megaton Hammer, or Biggoron's Sword if you have it, on his tail. Feel free to use the Light Arrows to stun him on the face if you wish, or roll through his legs.
Once you have dished out enough damage, you can stun him and get your sword back. Go back into the arena, and fight him with the Master Sword. Once you have done enough damage there, hit him in the face for a final blow.