Things You'll Need
Instructions
Stock up on Magic Potions before you go into the fight so you can use your magic spells to deplete Dark Link's life. Fill all four bottles with Magic Potion, if you feel confident that you can kill Dark Link without getting hit too much. Fill three bottles with potions and keep a fairy in the last bottle in case you get knocked out, if you're less confident.
Use Din's Fire on Dark Link as many times as you can. This is a somewhat close-range, area-of-effect attack, so you must make sure Dark Link is within reach when you use it. This battle has no time limit. Take your time and make every attack count. Don't waste your magic power or potions on attacks that don't connect.
Finish Dark Link off with a few swipes from the Biggoron Sword. If you manage to connect with Din's Fire until all of your magic meter and three Magic Potions are depleted, Dark Link should fall fairly quickly. Avoid using your regular sword to finish him off, because Dark Link mimics your attacks. Get close enough to Dark Link so that your attacks will connect but his won't. The Biggoron Sword is roughly twice as long as the Master Sword, so this should not be a problem. Keep striking until you win the battle.