I assume this was reflected damage because the damage i received scaled with the power of my attacks. When I rested next to him, i received almost no damage from his normal attacks.
I managed to finish him thanks to an infusion of acid explosion, but died a few turns later to the boss [i still had the original healing infusion and couldn't regenerate fast enough]
More generally, how do you deal with reflected damage ? Especially as a melee fighter ? I want to be ready for that guy when i meet him again