There is a lot of use of magic damage on that last boss. If you haven't already been doing it make sure to have Eos toss a fey covenant on the party. If I remember correctly you should have emergency tactics by that point. Emergency tactics succor is a nice AoE heal to recover from damage when no one is in immediate danger of dying if multiple people are very low then use indomitability to get out of the danger zone then emergency tactic succor to recover the team to more manageable levels.