This is why you don't use Z/L targeting. He parries vertical slashes and counters jabs. The easiest way to beat him? Slash. Turn around. Slash. Turn around. Slash. No really. He refuses to defend against horizontal slashes - it's so disappointing...
I would love a Dark Link fight where he doesn't just copy your swordplay, but also uses dark copies of Link's various items (i.e. Dark Hookshot, Dark Bombs, Dark Bow, etc). Make it truly feel like you're fighting a doppelganger of yourself.