Here's a guide on where to find, and how to kill, the third boss. But, if you would like a walkthrough.
1. I would first recommend buying time warp in Arkos to change night to day, and vice versa.
2. Travel to the bottom right of the map, Stormheim Shore.
3. Travel out to the platform on the water, and use time warp to make it night time.
4. Use the thunderstorm ability to make the castle appear.
5. Enter, and grab the jump up ability while you're at it.
6. Fight the boss.
- The boss has three attacks: A beam you can easily duck, creating fire you can easily back up and avoid, and tentacles traveling through the water you can easily jump over.
- Attack the boss in the middle of his chest, jumping twice maximum to attack and then backing up and ducking.
- Once health is below half, jump and attach once, then back up and duck.
- With all of this being done, the battle should be relatively easy.