Just like Uriel had taken his time learning about the merman weaknesses, their attack patterns and their way of thinking, the merman king had done the same thing and came up with a strategy that resulted in Uriel's defeat.
Uriel dropped on the shallow water with expressionless eyes, his face already pale from the blood loss ended up completely submerged beneath the surface of the water and due to the merman king's skill, no matter how hard Nika attempted to pull him up it was impossible.
Luna cranked up her level of brightness to the equivalent of a hunter grade flashbang and managed to blind the merman king leaving him stunned for a few seconds, but that was the extent of her power and it caused no damage to the vicious cryptid.