Phase One
During phase one, the boss's tentacles are sticking up out of the water and it uses these tentacles to slap your character. The only way to prevent yourself from getting hit is to shoot the glowing yellow orb at the end of each tentacle. If you successfully shoot one of the tentacles, the boss will move onto phase two. If you do not successfully shoot one of the tentacles, the boss will slap you repeatedly before moving onto phase two.
Phase Two
During phase two, your character moves to the left or right of the boss, depending on which tentacle you shot. If you did not hit either tentacle, your character moves to the right first. At this point, you must shoot the glowing yellow orb on the tentacle closest to you. Your character will then turn around and look at the fan. Press the "C" while looking at the fan to use your "stasis" move on the fan and slow it down. Your character will then run behind the fan. Once you are safely behind the fan blades, the boss will reach in to attack you, but will have its tentacle cut off in the process. You must repeat phases one and two for both tentacles.
Phase Three
After both tentacles have been cut off by the fan blades, the boss will go into phase three. Phase three is very similar to phase one, with the boss's tentacles sticking up out of the water, only this time there are explosive barrels to throw at them. Aim your pointer at the red explosive barrel that floats to the surface and press the "A" button to grab it with your kinesis. Aim the barrel at one of the tentacles and press the "A" button to launch the barrel at it. Repeat for the second tentacle.
Phase 4
Once both tentacles have been hit with an explosive barrel, the boss will go into its fourth and final phase. During this phase, only its head is visible above the water, with a small glowing yellow orb moving back and forth across the surface. The boss will shoot explosive balls at your character. When it does so, press the "A" button to grab the projectile out of the air, aim the projectile at the glowing orb and press the "A" button to launch it back at the boss. Repeat this process until the boss is defeated.