This sounds like a fun concept to work with. Haven't seen another idea quite like this.
From the very last image, I think the 2nd one from the bottom up (the one with the pink hair and angler-fish cannon) ties closest with the image of a champion for this contest. I think a mixture of medieval armour and punk aesthetics can make for a crazy aesthetic.
If her purpose is dragging down others to Davy Jones lockey, maybe the cannon can shoot chains or anchor weighs to achieve this? Just a thought. Love to see where you take this.