Dark Link is porbably more skilled than anyone who's played Oot. Using the Master Sword, Dark Link is nearly impossible to defeat. The only thing is: he doesn't have items. You can use Deku nuts to stun him and then hit him with the Megaton hammer. That's what I did. (It probably hurts too.)
The thing that makes him hard is that he copies your moves and can use your moves against you if you do a certain sword strike. But if you get past those obstacles in ways stated below, he is fairly easy to beat.
He can copy your moves, however, he doesn't have the items you have. I used the Megaton Hammer when I was fighting him, because it's hard for him to defend himself against that. The Biggoron Sword is also helpful, I hear.
I just beat OoT3DS some hours ago and Dark Link was incredibly easy with the Biggoron's sword! I hit him as soon as he "respawned" after a previous hit and he went down very quickly. On early N64 version playthroughs when I didn't always have the BGS I just spammed him with Din's fire... very easy.