A floor boss is a powerful enemy guarding a tower gate. You face the boss with its guards and can earn more gold than on a regular floor.
Check the approaching boss and your preparations under "Boss Gate" in "Tower". On that floor, read "Scouting Intel" before entering battle.
Rest your main party and review the front row’s equipment before a boss attempt. Your first victory over an abyss floor boss earns prestige, so use return visits to prepare a party for the next gate.