For her, you want to use All of her unlockable Jump related Unique Talents (such as South wind) paired with Double Jump. You want to especially do this since Jump Attack damage increases with height . Another main advantage of entirely focusing on Jump attacks is that it can also trigger a Perfect Dodge if you unlock Haniwa Horse Rider.
For unique talent you want to choose, you'd want the one that gives you more attack hits while disabling ultimate's burst. This is a worthy trade-off in the long run considering you'll also maximize crit with the Less HP => More Crit that pairs well with Mokou's 1 HP crit.

Her main weakness would be bad timing when you use her immortality skill, because some bosses would immediately trigger spell cards that makes them invulnerable for quite a bit, so use it wisely.

Youmu is the "fastest" character here but she doesn't have that immortality skill both Mokou and Flandre has. Luckily, you can compensate this with her fast attacks and her talents which snowballs into broken damage numbers that fills your entire screen. Focus entirely on basic attacks alongside additional slashes including unique talents related to additional basic attacks.

Maximize it and get additional energy recovery talent from blue points. since each slash also turns enemy projectiles into blue points. This creates a huge feedback loop of Critical Hits => Special => Ultimate => Critical Hits.

Of course, since Youmu can also be very fragile you also want to get more health from her (At the very least Max 150-200 HP for her).

Overall. So far it seems like each characters are playable. Feel free to add additional feedbacks and other things I'm missing.

And this isn't a guide that immediately makes you undefeated each run, sometimes you can just get bad timings or unlucky rolls but knowing the fundamentals of your character does improve gameplay peformance.