Ok, now that I have a bit more time I'll explain my reasoning.
Increasing CherryMax value is the main prerequisite for scoring high in PCB, so it is a useful skill to do better at the game. It does involve grazing, not getting hit during supernatural borders, strategic bombing for bullet canceling, and dying to replenish bomb stock and abuse max power bullet cancel. Which are all things most players normally don't opt to do. And here is their chance to try it and see what they can get out of it. Sakuya-B is arguably the perfect choice for it because she's pretty much tailored for this particular challenge, while her normal cherry+ gain possibilities are inferior to Reimu-B and Marisa-A (so she has to use bombs more often).
Now for Subterranean Animism. I know I could pick Reimu-A for this challenge, but I purposefully didn't do so for one reason: it's too easy to graze while invulnerable. With Marisa-C, however, you don't really get the luxury of being invulnerable, so you have to graze things the hard way and learn the best spots to do so (of which Parsee is hardly the most profitable due to lack of lives). Which, of course, will make it easier with every other character afterwards.
