I cannot kill my reflection, what am I doing wrong?
I have discovered in my 7+ times fighting Dark Link AKA Shadow Link there are two main ways to go about defeating him:
1) You can use the Megaton Hammer as you sole weapon. The reason this works is because Dark Link copied all of the items and weaponry that you had when you fought Phantom Ganon (which if you had been going along with the game chronologically, you should have fought by now) he copied your items and weaponry, and you had received the Megaton Hammer since that encounter. The reason not to use your Biggoron's Sword will be covered in reason #2.
2) You can use sword and shield. This only recommended if you want to spend the longest time fighting him, because if you use this method he will immediately start using what ever weapon you choose to fight him with. If you use the Biggoron's Sword he will count it as a sword not the Biggoron's Sword.
One final warning: If you are playing Ocarina of Time 3D, Dark Link will progressively gain weapon usage the longer it takes you to defeat him.
The Reflection (Dark Link) will get stronger the longer the battle is drawn out; to the point where at times he'll jump on your sword and cut you. So you want to end the battle quickly. Some things I do:
Use the hammer. No idea why, but when I first fought him it always worked.
Use Din's Fire to get some space. It seems that he'll either get hurt and drop back into the water or defend and get pushed back which gives you a reprieve.
Don't thrust. If you're Z Targeting and moving forward when you attack you'll thrust; this is when he jumps on your sword. At the start of the battle it's not a big problem since he just stands there before back-flipping, however afterwords he'll slash you before back-flipping.
Carry nothing but fairies. Unless I need a bottle for something else I always have a full stock of them.
As I said, the first time I used the hammer and it seemed to always be effective. However after I got more confident in my skills I ditched that method and had a straight up brawl with Dark Link with just swords and shields.