Yuyuko > Kaguya > Eirin > bombing games as scoring goes.
I think, for flawless survival, (NMNB) which seems to be popular here, Kaguya is probably one of the hardest - if not the hardest (although I think Remilia would still be harder than her) - since her fight lasts for a long while, due to the extra spellcards. Kaguya also has patterns highly affected by RNG, which many people don't like (me included), since facing those on a full run when all wobbly and stressed is quite a terror.
For the Eirin vs. Kaguya thing, I'd say Eirin is easier in all of the cases. She has much more static difficulty, after practice only her third nonspell is terrifying (maybe). Hourai Elixir PoC isn't as bad as doing all of the five Last Spells optimally and correct. For stuff like Apollo 13 supergraze, Omoikane's supergraze/speedkill and PoC'd Hourai Elixir on harder difficulties, you don't really see such feats on full runs. Risk-reward balancing.
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Good job, everyone.