Din's Fire, the Megaton Hammer, or the Biggoron's Sword make it a lot easier. If you use the Master Sword never Z-target him and just try horizontal slashes. Also the less health you start the battle with the less health Dark Link has.
You better just get the Bigoron's Sword. If you don't have it hit him with Deku Nuts and immediately to a quick spin attack (by rotating the control stick and then press , you should kill him with no problem.
Just either use the Biggoron Sword or Din's Fire and you'll have him beat in no time. Also, don't forget that Dark Link's health is the same as yours. If you have ten hearts he'll have ten and etc.
MW7 pretty much covered everything.....just stab at him with the Biggoron's Sword and you should do fine. Or use the Megaton Hammer; he can't do anything about it since he copies your moves but doesn't have the MH. Or if you're a real n00b and you're scared of going head to head with him use Din's Fire if you have the magic for it.
The way i do it is i just crouch with my shield and do the really fast jab, works all the time, when he tries to get behind you just flip around and to it again.