When entering the third wave, all Servants including backline will have their NP gauge drained to 0% and will be inflicted by NP Damage Down (20 turns, unremovable).
Note you CANNOT use the revive full party option (using 3 Command Seals or 1 Saint Quartz) in this quest! If your party falls, the quest immediately ends.
A detailed guide for this encounter can be found [HERE]
He Who Saw the Deep (Archer)