Things you'll need:
1) Din's Fire magic
2) 2 bottles
3) enough money to buy 2 magic potions in the Market in the past
4) You'll need to be at the guy with all this stuff
5) Full magic bar to start withSteps to beating Dark Link:
1) In the past, buy 2 magic refill potions and have them in your jars.
2) In the past, you need to get Din's Fire magic from the Fairy at the Castle after bombing the rock at the dead end.
3) Make sure to get healed up and you have completed the Water Temple to the part when you get to Dark Link.
4) Warp back to Lake Hylia and go into the Water Temple.
5) Go to the left door and kill the bats on both sides of the entrance.
6) Go up the Waterfall and go through the door.
7) In the next room shoot the spiders with your arrows.
8) finish the room and fill up any used magic by hitting the jars and getting a magic jar.
9) In the next room, you will fight Dark Link. Put Din's Fire in one of your slots. When you start to fight, lock on and put your shield up until he is kinda close and use Din's Fire. This will hit him. He will end up behind you. Turn around and lock on and repeat the process until he is finished off. If you need to refill your magic, use your magic refill potion that is in your jar. In the next room is the Longshot!
Credit: Nick a.k.a. Shock