You will fight Thoras in Kronos's Sickle in Chapter 12.

Thoras can be handled like any other plant enemy like him, but he's no ordinary plant monster and has a bit more to his arsenal. He's actually quite easy, while he does have some high leveled spells such as Tornado, Divine Wave, Sunflare, and Dark Devourer, he doesn't use them very often, and instead, acts like other plant enemies, and you can use this to your advanage, baiting his counterattack while using a special move in the air, causing it to miss, or just break him and attack.