An Archdemon's Dilemma: How to Love Your Elf Bride: Volume 7 by Fuminori Teshima

Since they're already all the way out in the ocean, Zagan's group decides to go relax on a beach. Everybody changes clothes, and the men are all dazzled by the sight of the women in swimsuits. However, things take a turn when an Archangel named Michael drops by to deliver news that a sorcerer who slaughtered one of the Archdemons has marked Zagan as their next target!