So, went and completed the demo again (mostly). This time, with 2 specific restrictions:
1) Win only fights necessary for the plot. So no random encounters, no bonus bosses...
2) No Bombs.
So, unlike the previous completion, it meant going into battle with a very underpowered team. No overpowered (and sometimes even "just useful") spells are available, too. So, since I couldn't just blast through enemies, my opinion once again:
Spells that afflict states are largely useless, because, from what I've seen, ALL main bosses in Makai (and Gensokyo, starting from Yukari) are resistant to Confusion, Blind and Burn. So Blind spell is useless, Flash is useless, Deaf to All but the Song is useless... And in random encounters you won't gain much by using those spells.
For some reason bosses like to counter elements they are resistant to. What's the point? It's not like players will use such ineffective spells, after all.
YuugenMagan still didn't impress me. Easily tanked whatever sparks Eyes sent my way, butchered 3 of them and Insect Lullaby'd remaining two.
Yuki&Mai were... harder. Much. I really learned to "appreciate" that Yuki casts Immolate as often as a Fireball. Especially when she started to spam it constantly for 5 turns, which was every 2nd attempt. And tons of element resists they had on didn't really help. (Why does Yuki have 1/4 resists to both Ice and Fire while Mai has 1/4 to Ice, but takes 2x of Fire damage?)
Empowered Sunny Milk also gave a lot of trouble - I even had to return to Kourindou and stock on Hi-Potions. Confusion spam is less dangerous than fire spam, sure, but with my attacks being quite weak, even 1-2 attackers confused was a big problem.
Bug: If players are afflicted with the miasma, die during the battle and are revived, miasma effect doesn't return until after the fight.
Elis was easier than both Yuki&Mai and Empowered Sunny Milk. One of the main reasons is: she doesn't start with multi-hit attacks and by the time she does use them, you may have enough MP&TP to Insect Lullaby her. Still, that's better than "multi-hit fire/confusion spam on 1st turn" or "make one of your attackers useless".
And at the end of Makai I had a solid level 9 for all characters. Yeah, didn't even managed to reach Tier 2 spells

Just like Colticide, Yuugen Maagan may be one of the coolest boss in this game, but it didn't impress me either.
Colticide said something about YuugenMagan?
Elis is hard but manageable, Rumia's useless against her, but Mystia's Mooned Insect will still tear through her reflect but her combo will not.
What combo? Insect Lullaby doesn't reflect. And all others reflect because they all consist of only magical attacks.
For me it was even harder than Reimu (which I ended up fighting before since I actually attempted the battle right after Yukari and failed), but it makes sense since it's an optional boss and you can fight it whenever you want. And the difference between their immunities is what makes it a very hard boss fight.
Huh, and what made it hard for me is that 400hp undodgeable (even with blind) danmaku barrage attack which is spammable and is more than twice as strong as every other attack from them, even with weaknesses (Lifeshaver doesn't count). Immunities didn't really affected me much.