I ran into the same problem because I had only trained in unarmed, and taking him down with your bare fists is not realistic, especially when you can't save and therefore have to repeat the entire battle every time you attempt it. I ended up loading a save from before the battle, going back to Bernard and training my sword/strength skill up to around 12. If you put Bane potion on your Herod's Sword and land a few hits on him (keep going until there is blood on your sword, then he should be under the effect of the poison), you can play defensively as he slowly loses health. I'm actually glad they gave him a helmet because the arrow to the head cheese was pretty lame.