The least fun boss: Nitori Kawashiro
The reason is the same — random deaths. The spell-cards don't seem to be the hardest, but how many times did I have to use bombs for no reason, and how many times did I ragequit?
Favorite boss: Kanako Yasaka
The entire Walkthrough stage leading up to the possibility of facing the goddess of the Moria Temple face-to-face strongly reminded me of the struggle in eosd, in a good way. Setting aside the fact that I played through it like a complete retard, I cannot help but notice that fighting her was fun; I'll specifically mention the last spell-card — "Mountain of Faith"

In itself, it is very easy, you just drift from side to side, however, there are two factors that gave me an adrenaline rush: first, it is the final trial, meaning you realize there's "just a little bit left," but at the same time, it is very long in itself, which in such a combination increases the chance of error and is generally cool.
Favorite musical theme: The Dark Side of Fate