I managed to beat the boss. My main strategy was to prioritize HP (items that give +maximum hp, members with heal spell, weapons that return HP on kill).
It might be cool if you could buy potions so the party can heal themselves during a dungeon run.
One bug I noticed: one of my members had very low health, but managed to heal himself a few times during the battle so he was still on the battlefield with ~8hp at the end. But after the fight the member was dead. I guess he became dead at one point, and at the same time healed himself.