hey everyone i want to talk about a secret boss in the crimosn fields. alright so this secret boss you have to do a few things to get to first raise all the banners in the battle zone. next you got to wait a while until you get to fighting the ten creatures. the boss will then say on the bottom of the screen "I see your banners" he will then appear almost to the exit of the crimson fields. he is a fire boss so fire people be warned
This is only one of the bosses. His name is Yakedo, and, like you said, raise the banners then get to the part with the battlefield. However, he says that he will fight you since you raised his Imperial Majesty's Banner. Yakedo drops a number of useful items, including a no auction 35+ myth hat and the death ninja pig pet.
However, a boss you should take note of is Youkai. He hides within a tower at the Tree of Life, and he is tougher then the Jade oni. his battle cry is "Fool! I will crush you where you stand!"(correct me if i am wrong) . Youkai drops a large number of useful drops, including: -Minotaur pet that gives a Minotaur card at epic -a no auction fire hat -No trade storm robe He also will give you the Sword Saint badge if you killed him for the first time, if you were wondering.
I know there are more secret bosses, but i am gonna let everyone else take a stand for themselves.