IN Marisa
Most of the fight is static, and Asteroid Belt isn't really that hard. I'd actually say that she's the easiest windows stage 4 boss to get good at.
Reimu, on the other hand, is random as heck, and when you're fighting her, you have to use either magic team or scarlet team, both of which are harder to use than border team. Some of her attacks are not that hard, but overall, I find her much harder than Marisa.