Wasn't even the boss, it was just the monsters tricking them. For fun. Then they even trolled the mage. A cruel trap, but not undeserved for these treacherous bastards.
Would've been an entertaining end, too bad they got saved in the end, and it was Yuki too. They don't deserve it since they'll probably berate and insult her anyway for not rescuing them sooner, or for not being with them in the first place. Considering what these scumbags are like, I can see them abandoning her in this dungeon to die to save themselves.