Instructions
Equip the iron boots. This will slow both you and Dark Link down, but is useful because this foe is hard to tackle at normal speed.
Throw a Deku nut to stun Dark Link, and then hit him with a jump slash. This only works once as Dark Link dodges future nut tosses.
Use the "Z" button to target Dark Link and thrust at him with Biggoron's sword to get in a hit before he is able to dodge it. If you do not have Biggoron's sword in your arsenal, you can hit this enemy with the megaton hammer. You cannot equip this and the shield at the same time, so be quick on your feet to dodge Dark Link's attacks.
Go through the door after defeating Dark Link and open the large treasure chest. Inside is the longshot.