The biggest problem with the dark knight fight is her grand slash attack, with deals tons of damage and easily break your armor, so every time you see her getting ready to unleash this attack(the stance name is Reel Back, that is when she is preparing the grand slash), try to dodge it or simple use withdrawn, using carefull attack in the block stance can be somewhat effective too. I dont know if you can parry it, but it is worth the try.
Now for defeating her, if you are a mage, just paralyse, oil and ultra fire ball her, its the easiest way, if you are a ranger, i recommend using your arrow when she is going to use the grand slash, you have time to defend yourself after that. For the melle combat, try using medium-height attacks, such as tempo attack, because she doesnt have medium height armor, it saves a lot of work.
So basically if you are not attacking her with magic, you need to have high perception to always see the next attack and high agility to dodge. As i said before, withdrawn works too and parry is something worth to test.
If after doing all i said you still cant defeat her, i recommend some grinding and upgrades for your weapon.
I hope you understood my explantion, english isnt my native language.