How about using Vile or Unholy damage, from the Book of Vile Darkness?
Unholy damage can only be healed by magical healing, but Vile healing cannot be healed unless it's in an area that's hallowed or concecrated (I think). So, he could be hit with a weapon doing Vile damage, and the PCs simply Can Not Heal Him.
Unholy damage can only be healed by magical healing, but Vile healing cannot be healed unless it's in an area that's hallowed or concecrated (I think). So, he could be hit with a weapon doing Vile damage, and the PCs simply Can Not Heal Him.
