Spoiler below! (Sorry I don't know how to hide it)
Correction: I found three sealed doors in the sea
I managed to kill all bosses behind the sealed doors except the one in the Valley of Flowers...The boss refresh per turn of 60k hp and have a total of 100k hp. The maximum I can deal is about 30-40k only by using resonance and chains with Zeiguld Various etc. Seems like the point is to gather all the materials to make the piece of equipment which gain all merit+5.
Any ideas to defeat this boss..?