That skill does [Dark] damage equal to your current health, so it will one shot without mitigation. The best way to overcome this is to equip [Ring of Resist Dark] on your tank, and give the rest of the party one or two [Dark Resistance Brand] each. Have your entire party wear some form of dark resistance as all of his attacks are [Dark] property.
Likewise, he also takes more damage from [Light] attacks, so [Light Brand] on your damage dealers works well.
That's actually only done by the (revived) Distortion of Amilitae. Every 6 turns, he'll disable either your skills, or your items, for 2 turns; it's the fight's unique gimmick.
He's not a pushover, but if you keep his accuracy down (extended Murk lasts 7 turns and gives -30% accuracy), your party topped off and the boss taunted, you should be a-ok. Use recovery items when your skills are locked, and skills when your items are locked.
For this boss, some [Light] resistance also works wonders. Predictably, it is also weak to [Dark].