i think you're talking about dark link. Dark link copies all of your moves. But he has no items. Use the goron hammer to smash dark link. But watch out, everytime you hit him he'll appear behind you. Repeat until he falls.
Somebody just asked that question! now i get to answer it again. You use your heavy boots and wear your blue tunic to go under water at lake hiylia you will see a door under water nd that is where...