Alright, I'll give this a shot. Before I go on, I think this is your theme song and as thus should affect all other opinions. Theme songs are good
Eliza: I think my neutral with her sucks, I think my reset game with her is basic, I haven't learned to call stuff out with Sekh yet which is important.
Neutral with Eliza is full of hard reads if you're not using lights or cr.MK. Try to keep yourself in a position where you can use those freely, and if not, perhaps consider using your Summons to get in along with your assist. I've found plenty of success with Eliza point with LnL, backing herself up with Horus.
Sehk is not a fast a reversal, but it is one nonetheless. You may use it where other people may consider block+assist, punish up-backing or just plain making a hard read. I haven't found too many places to do it successfully, but I haven't tinkered much either.
Big Band: I don't like or play this character, in fact, I hate this character.
He has quite a ton of damage, and Rush Punch + E-Brake is actually frightening for some. You can force people to jump and then punish accordingly. He's useful but also very straight.
Squigly: My input finesse isn't up to par for this character, I can't do low into high consistently and so my vortex is weak, neutral game is rough, I'm not good at sing tactics.
You won't actually need hi-low unless you're extremely confident often enough-- don't stress yourself with that too much. Remember you can seria cancel with PP and KK if you have it charged. Neutral with her is also interesting. I think you may want to watch NeonNightRider play at our locals-- he's very consistent with her.
Using sing to get into range and then perhaps SBO, cr.LK, cr.MK and instant divekick are things you should try out!
Filia: Her neutral game is rough and I don't understand it outside doing ringlet a lot and some IAD pressure, I'm not smart enough for this character. Drop combos too much.
Her neutral game is to get in. You need to make reads with RINGLET SPIRE but once you do, you're rewarded with a full combo! Filia is very good at staying in, and even better when you understand the rules of Skullgirls.
You want to be as random as possible and keep them guessing until it's too late. High, Low, Throw, Left, Right, Under, Over, whichever you can manage. It's not about being smart, it's about planning ahead ;^)
Fukua: I can't play her the way I want to, which is go in. If you ask me to keep away, I seem to do alright, but I'm not interested in playing keep away.
She's neat. Use her projectiles as a means to get in, and try to occupy the mid-range until you're confident enough to call a shadow to make them sweat, then go in as hard as you like. You have cmdgrabs when you ARE in there, and the ability to pressure them to come to you. Try to create a nice back and forth with it: Poke until you're in, use your projectiles and tools to keep them in and deny them an out and run your mixups.
Cerebella: I suck with her on hitbox, IMO. I'm just autopilot kanchou resets and doing random shit. Can't 360 reliably. No real low/throw game.
I can't say much about Hitbox. Don't be scared to low into qcf+throw, don't be scared to runstop, don't be scared to instant clap, and definitely don't be scared to Kanchou 360. Watch MikeZ play at casuals-- just make them sweat once you get in, because she's excellent at it.
Peacock: I'm ass with her. I haven't played her enough to understand her improv zoning game which is where I imagine she really shines. I drop combos a lot.
Can't actually say anything about this. Try watching Infy's old videos-- they'll still apply and I feel like you can benefit from learning Peacock just for opponent study.
Valentine: I want to say I get her neutral, then I get in a match and ahaha, right. I'm good at using level 2, whatever that means. I play her too much like Filia. Disappointed in myself.
I have 0 idea what to do with Valentine. ShinAtProof is my go-to example because I don't know popular val's other than Duckator.
Painwheel: Too. Much. Work. I cannot 9Fly on hitbox. Otherwise.. very fun?
I have 0 idea what to do with Painwheel. Weatherbee is my go-to example because I don't know popular painwheel's other than Elky Dori or Taluda or whichever?
Fortune: Drop all my fiber loops, not sure I understand the neutral head on, not sure I understand it head off, head off is too complex for me.
Play mixup. Learn headless. Focus on making the head do all the navigational and footsie work for you and use the body to confirm into a blockstring or a mixup-- you'd be surprised at how useful Headless 6HP > Throw OR iad J.LK is .
Double: Fun when she's in. I'm ass with her.
Double is almost the same gig as Filia or Fukua: Great when you understand your options and if you're willing to take what you can get. The difference between anyone else and Double is her confirm potential is ... straightforward? Focus on resets with her and perhaps a barrel loop ender combo. I am not one to ask for Double advice but I could see you playing her if you play her as a "Comeback" character as designed. Luger / Car / Monster / Cilia Slide / etc are all useful and work into Blockbusters.
Parasoul: The one character I've never dropped no matter how many times I've threatened it. I feel like I, again, don't really understand her neutral game. I just go in and press buttons. I don't think. I rely on: the fact that jLP is good and jHP is good, spiral flare (always gets blocked now), napalm shot spam, pillar/bikes, air throw range, and risky business. (fHP into sniper at round start, etc. Gimmicks.)
I have to fight so many fucking Parasoul's, man, I can't understand it either. I actually think you should watch 3rd Strike Urien or Ken videos and see how they play the game-- now play Parasoul this way. Otherwise, I don't have any actual useful advice and I'm sorry.
As it stands, I may be missing the question, but here goes: I think you should focus on the base hi/low/throw game instead of combos. As a person who is terrible at inventing combos, it is evident to me you can make them on a whim, and they will be useful once you understand the natural flow of the game.
Try to see if you can focus on just getting hits often and consistently. Once you get that threat of mixup in, your natural play will EXPLODE.
I usually am not scared of being hit by you in a match due to you doing fancy stuff but not really getting the initial hit in-- once you do that and get assist pressure going / stuff that makes me scared to retaliate, you're gonna go far.
I make no claims to be better than you nor do I have the most useful or correct advice for each part of this game, but this is what I have for you! Please ping me if you have any concerns.