[Octopath Traveler 2] Which post-game boss is harder?

Just finish the game and get ready for the post-game super boss Galdera. I saw that there are additional Extra Battle to fight against True Vide and Traverlers from the first game. I was just wondering which battles are the most difficult and which are the easiest among Galdera, True Vide, and Travellers. (I haven't tried any of these yet.)

Also, any benefit or reward for taking down these bosses in Extra Battle?

thx ya