The twelfth Colossus, Pelagia the Great Sea Beast, is a massive creature sleeping at the bottom of a lake. As soon as you swim to the center of his lake, he will slowly surface and roar at you. Although sedate, Pelagia has one dangerous means of attack: A pair of curving horns on his head shoot electrical bolts, which can cause severe damage.
As soon as Pelagia emerges from the water depths, dive underwater and swim. He likes to start the battle by firing a volley of electricity at you, which can be very damaging if it hits. Being underwater seems to stop these shots from reaching you. Ignore the floating structures for now and concentrate on circling the colossus so as to scale his back. Pelagia will turn constantly to keep you in view, so it may take some time to swim around him. Climb up the furry trail all the way to his head, where you'll find what appear to be a set of rotting teeth lodged into his "crown". These teeth, when struck by your sword, actually allow you to steer Pelagia left, right, and forward (there are separate groups of teeth in each respective direction).
Locate one of the floating structures and line Pelagia up with it by hitting either the right or left tooth sets. Rush him towards said structure by repeatedly hitting the front teeth. Once he's really close to it, you should be able to jump from his head to the roof. There is a small pillar you'll want to hide behind because soon after Pelagia will take a few shots at you. However, when he lifts his front legs out of the water and onto the roof, come out of hiding and jump onto the furry area of his belly. You'll find his only weakpoint glowing there. He won't give you much time, but you should be able to pull off 2 to 3 fully charged stabs before he submerges again. Repeat until he's finished (shouldn't take more than four passes).
If you can't out-maneuver Pelagia, there's another way you can get up to his head. Instead of ignoring the structures, swim to them. Go inside and the Colossi will move near it (you need to hide behind the plinth, in case your opponent decides to fire some thunder at you). Once he's close enough, use the ledges on the outside of the pillar to jump and grab onto his face. You can leap up his face this way to reach his head.
- This boss is no different on Normal or Hard Modes