I'm learning Harley as well (still trying to decide on a main
), and even though no one has posted on this thread for a while I figured that I'd throw my 2 cents in.
Harley's mix-up game is pretty insane, especially on KD. My go to BnB midscreen is b2,2-> TS(1)->ji2->1,1,2xxSS, (Corners I use b2,2-> TS(1)->nj2->f1,3->1,1,2 you can xx intoSS here if you want a tricky cross-up situation that leaves you in the corner -- pretty decent after you corner combo them a couple times if you ask me). The starters are either b2,2 (low, mid), or f2 (overhead), or ji2 with either of them (overhead low or double overhead).
Canceling 1,1,2 into SS leaves you with an untechable KD and youre right on top of them. So you can mix them up with any of your combo openers (b2,2 or f2 or either with a nj2 beforehand) forcing some pretty nasty 50/50 set-ups and fuzzy guarding situations that all lead into a 30sih% full combo. Timing a b3->ji2 instead of just the ji2 makes the combo do around 40sih%, but it's too annoying to do online for my taste since Harley's weird jump arc and short range on her 1,1,2 make the follow-up after the bounce hard to connect... it's pretty good for forcing a stage transition though.
If they insist on using a wake-up that stuffs your jump in or starter, drop the SS after the 1,1,2 and do a MBf3 instead. It will connect as a meaty after the combo and will stuff their wake-up... MB might not be necessary against some wake-ups, but I haven't messed with it that much.
To get her mix-up game started is another issue however. Her jump is pretty bad and she doesn't have free armor to get in. Her dash is really good though (fastest recovery time in all the game iirc) and SSxxTS is a super fast low that has crazy range (be careful against projectiles though since you will get knocked out). Harley relies really heavily on a decent understanding of the range on her moves and an effective footsie game in order to get her mix-up game started, so playing her a lot to get used to her normals will probably go a long way to making her seem more devastating. Also, don't be afraid to zone when you can/have to. Her guns are pretty fast and MBcupcakes catches people sleeping a lot, often forcing a sloppy rush in which makes getting your rushdown mix-up game started that much easier.
Just somethings coming from a newb that might make Harley's weird playstyle more coherent. I'm having trouble with her AA options, since her d2 and her AA guns are kind of... not good. So far what I've been doing vs heavy jumpers is creating space and stuffing big jump-ins with cupcakes and psychic j2's, but that's not exactly a solid strategy