Everything on Hard and Lunatic:
Easiest
Stage 1: Wriggle. She's a joke on every difficulty. Lunatic Wriggle is about as hard as Normal Yamame.
Stage 2: Parsee, I guess. [Large Box and Small Box] can get claustrophobic at times, but nothing she throws at you is actually hard. Mystia's pretty simple too, I don't know why everyone hates her last midspell so much.
Stage 3: Probably China.
Stage 4: Aya with bombspam Murasa, probably. Only her survival card is really difficult, and even that is just memorizing a path for yourself.
Stage 5: Reisen. Lolmemorization
Stage 6: I don't know how, but I randomly just got like six times better at fighting Yuyuko. I have no idea how, but she's really easy now.
Extra: Suwako, except for that fucking second survival card.
Hardest
Stage 1: Tough decision on this, Yamame vs. Nazrin. Nazrin's manageable until her last card, which is utter bullshit on Hard and Lunatic. Yamame's last spellcard is an autobomb for me on H&L, and her opener on Lunatic, oh my god.
Stage 2: Kogasa ;_; Her last two cards are autobombs because by the time I reach them, I'm so low on power that I'd rather bomb them than time them out >:
Stage 3: Yuugi. Mt. Ooe is just insane, and I still have no idea how to do her last midspell. And losing a life to [Shackle a Criminal Can't Break] because you were trying to get the life piece is just humiliating.
Stage 4: Satori. All of them. All of them except ReimuC, she's really easy.
Stage 5: Shou, but mostly because CURVY LASERS and I can barely reach her on Hard, so I don't get to practice much. >:
Stage 6: I haven't played Byakuren on hard, but I can imagine her being the hardest. Scarlet Meister, though...
Extra: Flandre. I've gotten her down to 1DNB and she's still the hardest boss.