OK so they didn't kill the princess because she was a political hostage, I can somehow get around that.
But why the fuck didn't MC take the flute with him and maybe put it in Luxions main body? This sounds like a deus ex machina to easily let things get worse.
After that transparent film joke, I thought the author would pull a "This is now a Manga where you always win the first price in the raffle".
Sadly it wasn't like that.