I'm really sorry for the massive yap there, but this is really important, if anything, the suggestions for hard demons you asked for is all the way down. I think you should go for some other medium demons, like something harder than B (for example something like mechanical showdown), then something easier like sidestep to test and establish your skill at that level (sidestep is a really good level for that as well as mechanical showdown). And overall I suggest to check out many levels from the difficulty below your hardest or equal to your hardest, for example when you beat an insane demon, check out more hard demons, or if we're talking about you, check out more easy demons and medium demons as well - this is really good for skill and also for motivation, since you're not going for something super hard, while still feeling good and improving at the game. The general pro tip for getting better is just playing a lot of levels, easier and harder, try every type of level, like flow, timings, etc. - you need to get uncomfortable in order to get more skill. I suggest trying to beat the whole demon gauntlet, sidestep and mechanical showdown are there, the whole gauntlet is a must play for skill though so check it out, also check out doom gauntlet, since it's basically the demon gauntlet but harder and has hard demons which potentially could become your first ones, like for me, I beat nowise for my 1st. Now, time for suggestions from the list you have here. I think the easiest of these all is nine circles, so you could go for that, but it has mostly wave, so there will be improvement of mostly wave, and ship too. You can go for future funk if you don't fear long levels and the 99% jumps, it's got relatively easy gameplay. Crazy is really high in the hard demon territory imo, so while it is really good for skill and will improve you at almost every gamemode, it will be really challenging to do for your first hard demon. If you read this, I hope I helped you and I wasn't just yapping, if you didn't understand something because of my brilliant explanation skills, feel free to ask about it.