Puppen's doll bullets take small chunks off the creature's head and torso, which is then incinerated by Samson. As more of the body appears from underwater, it, again, is destroyed by Solomon's flames, and even after that it gets damaged by Abarim's punches. As Secundus floats over the lake, and just before his explosion completely destroys the parts of the creature that are above water and drops the water level by a good dozen meters, he sees something very shiny at the bottom.
However, rather than immediately resurfacing, the creature, which now appears a bit smaller and slower, stays underwater and hits the ground from under the opposite side of the lake with part of its body. It seems to have been doing this for a while, as near immediately a large section of the ground falls into the lake. After all the loose dirt and silt rapidly settles, it is plainly clear that the creature is larger than originally, as with the lake.
While still submerged, it starts pushing the entirity of the lake against the bit of land everyone is standing on, creating massive cracks in the ground which instantly fill with water, which seemingly only accelerates the process.
Crunkledunk learns that this is a "Kelpie", a sort of water spirit. It directly draws power from the size of the body of water it inhabits, and, despite likeing aquatic habitats over any others and rapidly drying out outside of them, doesn't actually need water to survive. Instead, all it needs is a sort of pearl-like stone that allows is to recreate body segments so long as it continues to be within its resident lake.
Its actual physiology seems to be mostly water, which is itself created from a unique, highly abundent form of magicule, making actually harming it with anything but fire extremely difficult. The most important thing for the current situation is the fact that there's only so much "water" it can move at a time, despite appearances, and that when doing things like what it is now its "pearl" is vulnerable.