As a healer in this dungeon, your main focus is to protect the tank no matter what. If you notice other party members' health is low, ask them to gather near the boss so you can use Clay's Salvation to heal everyone.
Apart from healing the tank, you must also remember to use Curse Explosion along with Touch of Despair. I didn't showcase it in this video, but it's crucial that you do. When the boss raises up, start by applying Touch of Despair to the boss if it hasn't been done already, and then apply Curse Explosion. If Touch of Despair is already on the boss, it provides a Health Shield of 1,000 per stack to an ally you select for 3 seconds and deals 100% of the total damage you dealt.