I just realized that the Prismrivers are totally color coded to the main characters.
Marisa fights the Red-White sister
Reimu fights the Black-White sister
Sakuya fights the White-Blue sister
I'm so glad that I just realized this, it's way easier to remember my Stage 4 Route as "Reimu fights not-Marisa, and then I have to focus fire on not-Sakuya / Merlin in the group spellcard". (I also don't have to remember any of their names this way. Except for Merlin. Merlin is way too weird of a name for me to not remember)
I'm kinda glad to hear I'm not the only one who struggles with Youmu and the Prismriver Sisters.
Yeah I wasn't expecting the second half of PCB to be such a gauntlet. Stage 4, then the Prismrivers, then Youmu then Yuyuko and her troll extra phase are really exhausting. Tbqh I'm not sure I can clear this game, I've only cleared other games by learning the first four stages perfectly and coasting by with tons of resources. PCB is way too honest for that though lol