In order to deal with Xiaoqing, the Sea Devils set aside their previous feuds and contrived such a trap.
It is known that within the Sea Clan, various races harbor enmity toward each other. They only reluctantly unite under the pressure of the Dragon Tribe, knowing full well that in a world where the human race is thriving, they must stick together to survive.
Normally, both sides deliberately avoid each other to prevent conflict from erupting.
Now, because of Xiaoqing, they have actually set aside their hatred temporarily; one can imagine just how great their animosity towards him must be.
The trap set by the Sea Clan is not the only one; there are several others in different locations.
As for why they didn't ask a fifth-order Great Devil to take action?