This bossfight is quite interesting. You will mount Sidon, which will take care of encircling the boss and navigation through the battlefield. At the beginning of the fight, the priority is shooting the big, ice projectiles flung by the boss. You have to shoot them while they're flying - if they hit you, you will receive minor damage. When you shoot down the first wave of the projectiles, Sidon will close in on the beast.
When you're right next to the water stream, jump into it, fling into the air and shoot shock arrows at the pink spot on your opponent. It will light up in green, and you will get back to Sidon to repeat the sequence as explained above. Take down projectiles, use the waterfalls to jump onto the enemy, and shoot weak spots when it's exposed. After a couple rounds the enemy will be defeated and you'll be able to continue.