You can find more specific details about the quest here: 💫 Memento Rare Quests
Also, specific strategy for this quest - the bosses do not apply any debuffs or DOTs, so don’t waste a slot on antidotes. They also hit everyone with their specials, so riposte/counterattack heroes are great to bring, especially Boril, as @Darth_Fuego pointed out. I actually doubled up and brought both Boril and Cyprian and just breezed through the last level.