At a high level, it seems that Forsworn Briarhearts get ridiculously tough to kill. I was level 60 or so and fancied myself invincible when I ran into a group somewhere in the Reach. I dispatched the Forsworn with comical ease.... until the boss room. I entered the Briarheart's lair full of fury and bloodlust, expecting a quick victory, but he calmly looked up from his alchemy table and proceeded to demolish my hapless character, firing two ice spells before killing me with ONE vicious power attack. It took three more attempts before I had the sense to use one of the devastating poisons that I'd collected over the course of my journeys to tilt the odds in my favour. It hasn't been the last time since the Skyrim balancing/levelling system has surprised me in a nasty way.