Child of Light

This is unlocked after the boss at the end of the Temple of the Sun. My party was levels 45-47.

The boss is a water dragon, which means it does water damage and takes additional damage from lightning attacks. It has two tentacles, which move along the cast bar very quickly and cast quick spells for light damage, and will often interrupt your casts. The boss herself does heavier damage, but is much slower.

I used Aurora, Finn, and Oengus for this fight. I equipped the melee fighters with the highest rank Emerald Oculi that I had so their attacks would do extra lightning damage. I also equipped Sapphires in the Armor slot, so they would take less damage from the boss's water spells.

Start the fight by burning down the two tentacles, since they are very fast and interrupt your casts. Once they are down, simply hit the boss with your strongest lightning attacks. Keep yourself healed as needed. Even with the Sapphires reducing water damage, some of the boss's attacks could do 150-200 damage, so keep yourself above that mark.

If you are having trouble, don't be afraid to use your consumables. The final boss fight is immediately after this one, so you don't have to work about conserving them.

As always, please comment if you downvote so I can update and correct the solution.