Onyxia

The Serpent of the Cold Depths; Onyxia is one of the Frost Mountains Colossal Titans you'll come across in Titan Souls.

Tips for Beating
You can manipulate the ice platforms that float in the water by shooting an arrow and pulling it back in. When you pull, the ice platform will pull with you.

This underwater dragon will hide out before popping up trying to electrocute you. The electric attack only hurts you if you're in the water, and it creates more ice platforms. You'll need to stay off the platforms and swim until right before he starts to attack. Wait until the perfect moment and target the purple tail with a single hit to defeat.

On hard mode, there will only be one ice platform at the start of the fight. Simply wait on the ice, and he will create another platform on both sides.