As Reimu is at the end of the game the easiest way to defeat her is by spamming her with the thousand daggers skill. This should be done alternating between time stopped and flowing as to not run out of time or mana. Having time upgrades and mana upgrades are also helpful to keep you in this state, and it’s very difficult to die while also dealing immense damage to Reimu.
Leveling up also helps a lot and will be even more useful if going on toget boss rush at rank S. The best high level spot to farm is the long hall right outside of Reimus fight using the same thousand daggers spam method.
The thousand daggers skill used for this solution is found in a secret room on the far right side of chapter 3 connected to a 2 story room next to a solo save point. If you have trouble finding the block to access the room, the auto aim skill can help find secret rooms.