Thorn and Kalus:

Now that you've passed the dreaded, forgotten forest, swam through the debths of the water temple, climbed the highest peaks of the snow temple, beaten your way through the molten canyon of Thorn's Pass and reached the bridge of Thorn's airship, the final battle with the dreaded Troll king and Kameo's fiendish sister Kalus begins.

Unlike the other bosses of the game, these two can be defeated with any elemental warrior, though I reccomend using Thermite and Chilla.

To make things easier, use the eye of strength(which is found by blowing up the Thorn statue inside of the chamber of living portraits with Thermite) and use your egg bomb attack. You can also suck certain enemies(which Kalus will spawn onto the battlefield every now and then) into Thermite's egg if you have upgraded him properly.

Eventually, Thorn will grab onto a long chain with a spiked ball attached to it. Just back away from him and continue to use Thermite's egg bomb attack until Thorn is down.

Though you may think you have won, the battle is not over yet. Once Thorn is down, Kalus will begin to heal Thorn's body from above. To defeat Kalus, transform into Chilla and fire your ice spears at Kalus until her health has reached 0%. However, in your attempts at defeating Kalus, every type of Troll brought into the game will be after you, so take her out quickly. Once Kalus is down, the end cutscene will begin. I don't want to give much away, but it hints a sequel.