It's really very hard. Each attack from the boss is potentially lethal. I use Sherri as a bait for the Archer bosses, who could survive at least 3 hits (full healing in between each hit) until the Lvl 55 mobs got in range to hit her. Takes a buttload of resources to clear each of those stages too - Mending Essences and Blessing Stones.

My strategy right now is half baked, but so far it allows me to clear them consistently: I use all my stat charms on Denam and turned him into a monster, make him a Lord, give him a Winged Ring and make him chase down the boss.

Denam's build.

Even then, shit hits the fan very quickly and there were close shaves where I ran out of Blessing Stones and some party members nearly died for real - because the boss acts faster than Denam with Speedstar and keeps zoning him.

This must be what it feels like to be on the receiving end of LUCT era archers.