Instructions
Run from the middle platform and jump onto one of the corner platforms as soon as the final fight starts.
Target Ganondorf and draw the Master Sword --- or, if you have it, the Biggoron's Sword.
Wait for Ganondorf to fire an energy blast at you, then swing your sword just before the blast hits you to reflect it back at Ganondorf, who will reflect it right back at you. Continue this little tennis match until Ganondorf fails to reflect the energy back at you and is struck by it instead.
Switch to your bow and Light Arrow and quickly shoot Ganondorf. A hit will stun him and give you a window in which to attack unimpeded.
Jump back onto the center platform and hit Ganondorf as many times as you can with your sword. Eventually he will recover, and you'll have to start again with the tennis match. Continue this pattern of tennis matches and sword strikes until Ganondorf transforms into Ganon, his second, more monstrous form.
Target Ganon's face and shoot him with a Light Arrow. This will stun him and allow you to get behind him.
Draw either the Biggoron's Sword or the Megaton Hammer. (Ganon has taken your sword away by this point, so you'll need to hit him with a different weapon.)
Attack Ganon's tail with your weapon until he recovers from having been hit with the Light Arrow. Dodge away from his sword swing, then draw your bow and fire into his face again. Repeat the "shoot-slash-dodge" pattern until a cutscene happens and you recover your lost sword.
Switch weapons to your best sword and continue the shoot-slash-dodge routine. After he takes enough damage, Ganon will collapse, and you'll be treated to an immensely satisfying cutscene of Link finishing off the giant blue pig for good --- at least, until the next game.