Unkillable? Are you sure? What combination was that?
combat master - so he was invulnerable regular attacks and finishers
and he had the one were killed beasts quickly
He was vulnerable to fire but fire arrows count as a ranged attack so that didn't work, exploding a campfire only took off about a third of his health.
Also he was the first captain I ran into in the game so he killed me a bunch of times before I realized what was going on so he was also at level 20 before to long.