If memory serves, the idea with that boss is to focus on healing between attacks. Try making sure you attack with an advantageous phase, then shift back to a defensive one by the end of the turn. Doing so will also heal you in the process, so it should be possible to stretch the fight on pretty long by playing conservatively with your cards.
Of course, if the damage is too high, you could also just attack while staying in a defensive phase, and just take the fight slowly.