'This is such an awesome question', I thought to myself when I first read it. Its still awesome so I've been spending some time thinking about what my pick would be.
Stage;And if you had asked me a year ago it would probably be something UFO related but through more recent developments I've really grown to get pretty tired of SA's Stage 5. Only the stage, not the boss nor midboss for that matter.
The part of the stage that comes right after midboss Orin. The pellet hell. This section generally only has one tool to mess you up with. Bullet density. I'm probably making this out to be a bigger deal than it actually is but I find it really infuriating to die simply because you run into a star that you very well knew were there but you still rammed into because the gaps are so tight. The part following it also seems to be nothing but lazy memorization nonsense that requires you to do things just right.
I'm not saying that the stage is BS. I'm saying that I don't approve of the way the stage relies too heavily on pixel perfection and memorization with no real room for improvization. And that is unfortunately a kind of crutch that SA likes to use a bit too much rather than designing an interesting kind of challenge. Oh and its also pretty damn unfair to have anything before the midboss be a mindnumbingly boring tediumfest. :V
Boss:Kaguya.
Seriously I hate that damn moon bitch. All her spells are seemingly designed to piss me the fuck off which is why its awesome there is something called FinalA... except you lose out on major points. Even though and yadayadayada i'm here to whine about the boss.
So, first card and all non-spells; No issue at all.
Second card, sometimes giving me unreasonably little room to maneuver when it comes to dodging the stars. Apparently you need to abuse the fact that hitboxes on lasers turns on later than their sprites. Fuck. That. Noise.
Third card: Lame ass streaming but nothing really soulcrushing.
Fourth card: Kaguya flying into you, Kaguya caging you and butchering you with a wave of red bullets. Kaguya forcing me to dodge through something that almost looks like a wall!.
Fifth card: Its fucking Hourai Jewel. I don't need to elaborate on this PoS.
Why do her non-spells even EXIST?
Yeah they suck. Especially with Marisa.