When you fight Zant, if you notice he teleports you to a battle arena you previously had to fight in. (example) The first arena is where you used the boomerang to kill the boss and get the first shadow piece. When Zant is in the air, use the boomerang to knock him down. Then start hitting him with your sword. Remember, when you already had to fight a boss, use the weapon on Zant you used to fight the previous boss.