I think that with all the ways a frenzied barbarian can bleed himself/herself to death, that the PrE needs a capstone that lowers the damage they take from their own attacks, or heals them somehow. Maybe a passive regeneration effect? Like 1d6 every 20 seconds, and increased healing amplification when at low health, similar to how "Don't Count Me Out" gives PRR when the user is under certain HP milestones.