Hmmmm the only other way that i can think of making sure that you hit the DK, is to invest some points in the Whirlwind skill...
You cant really miss with that... and it has the ability to stun the DK.
If you didnt learn it, the Ranaar student in Act 4, Academy Level 3
Map No 5
teaches the skill.
(yea unfortunately if you didnt learn it, you will have to load a save <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" /> )
WHat can also make a huge difference, is check the stats on your weapon, what is your acuracy and speed penalty?, try with a different weapon and see if it makes a noticable difference.
Also the fact that your DK was a ranged and you concentrated on his agility will make him harder to touch though <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" />
You will have to have something like poison weapon skill aswell, while you dont hit him every time, he still takes constant damage at least.