I was looking through this (and some youtube video series) to figure out the remainder of the normal game's bosses, but the guide doesn't go into detail about post-game bosses and the video series didn't make me feel too confident in my understanding of them either. Gisvarg is the secret final boss, and it seems like you need to defeat the 4 spirits (light & dark not included because they work differently?) to get their respective orb in order to unlock Gisvarg's dungeon. I still don't know about monsters like the Dragon Lord, Zenith Dragon, Celestial Beast, the seemingly random "boss + 2 minion" combos you fight at the end of every color dungeon, etc. And there's some bosses taken from older games like Marquis de Leon that seem to appear as regular monsters in Caravan Heart, so that's throwing me off too.
I just would appreciate someone who's well-acquainted with the game to list out the boss order clearly for me, noting when one may be optional or randomly selected from a pool of specific monsters, however it works.