Tips to Increase Score (More Spirit)
The more spirit you have the more score you will get. But theres more mechanics to get a better score.
One of the game mechanics is that there are type advantages. Every card has a type. Em (shown as a red setting sun), Ro (shown as a yellow halfmoon) and Co (shown as a blue star).
Em is bad against Co. Co is bad against Ro. Ro is bad against Em.
I have fought with a level 20 Ro card against a Ro boss and my ranking was always D. After I tried a level 5 Co card my result improved to a C.
Theres also the effects of the cards and characters that are important. Reimu and some cards give you more score if you reach a certain combo count.
The most important thing should be the card type and spirit.