Since I don't have much time left to finish something for this event I wanted to stick with whatever comes to my mind first.
The idea I had, was to have a colossal beast in some kind of diving suit using a mounted harpoon as a weapon.
Although I really like the idea, it does not support the idea of the elementals, because in this case water is nothing more than its habitat.
Sooooo... it's back to the drawing board I guess 
EDIT:
I kept the general idea of the title and thought of a creature living in the depths of the waters. However, I switched the character's element from water to lightning and wanted to go with a Jellyfish as the overall concept. I did some sketches to explore this idea.
While humanoid animals are not exactly the most innovative idea, I like how the tail of the jellyfish transforms into the character's dress in 1 and 3.
The last two were just some goofy ideas I had where 5 is a mind-controlled monster looking like a jellyfish and number 6 being a jellyfish controlling a mech.
Tell me which one you like the most while I continue to elaborate some of these ideas 
