@ Demon DK
You are playing your DK like it was a warrior. DK's aren't warriors.
Read the second essay in here:
Once you realize you aren't a warrior, you should prioratize your defense so you can cast more spells. Sure he doesn't knows whips, but you are throwing one of your defences out the window when he can already smash the other two.
Just wield a cursed polearm. You probably malformed a carpter hammer. I would use it. Even if your whip is hiting the double of times, your polearm will do the double of the damage and defend a lot better.
Next look at him and check his saves. You can use fireball to check saves. Crap fireball damage == high AFF saves.
Your best AFF spell is Iceball.
Get a decent pet:
You can get the Warlock to auto-lag him so you can cast and prevent him from fleeing.
You can get a caster pet to cast spells, like the Mistress of Magic or Torilar.
You can get a dirt + damage pet like the dwarf dude, crab or lion pride, but you can't really cound on pet damage vs a crusader.
Enlarge yourself.
Read the essay at the top.
On Behead/Decapitate thorycrafting. I know not what it does. I always tought it was a Cleave Mk.II . A high damage opener that probably can't be used in combat or blind. So if it gives you to much trouble, try dirt kicking him. Like mino charge, that lion pride auto-dirt can save you a lot of pain when you flee. I have no idea why you weren't shield blocking and I find it very strange, but I think it has nothing to do with Behead.
If you look at your second behead, you aren't also parrying. Most likely he is full on hitroll + offensive bonus. So polearm or if you insist in whip, dual wield them. Because if you can't defend there is no point in not using the most offensive weapon on each hand.
PS: there is a magic cursed whip in the bathhouse.