The only other thing that MAY help in this case, is to invest in about 230948203984029384903289 scorpion traps.... and some summoning dolls...
The DK (by default) focuses on you, while your summoning dolls can have some fun, along with the scorpion traps... the DK will unsummon some scorpion traps, but if you cast enough, they will get him down... but be warned, he will STILL focus on you, so have loads of potions ready...
If you have a save of just before you got to the end, unlearn everything in the DK's skills and invest those points on "useless in battle" skills, like identify, alchemy etc...
I also suggest that you invest some skills in critical hit... if you get a critical on the DK, he is stunned for a few moments.
