Game developers always, always, always make their games too hard. It's because, if you made the systems, poured over it for dozens of hours, you will always overestimate how well people do at the game. I had to grind the final fight until I got lucky with my "sacrifice" card stacking with a x2 bonus. I don't think there's any other way to win that fight.
The game would be tons better with an 'easy' difficulty. Other than that this is an amazing game!