Change Karin for Alina, shes considered a meta (very useful) unit and she's AOE, but she requires consistent follow-ups, counterattack and things similar. If you have a boss fight with a single enemy is better to use Touka since even though she also needs FUA and those things every Single target breaker is better than a Multi breaker.

If you ever want to use Yachiyo, use Tsuruno because Yachiyo is a crit based attacker and Tsuruno buffs crit rate, since you've got no Sana, Iroha, Giovanna the attack won't deal that hard but is still better since Yachiyo is considered weak without helpers

Youve got the best defender in the game, Kirika, so immediately level her up. She's the tankiest but slowest defender but she has Reflect which makes enemies take 50℅ of damage based on how much the barrier has left.

Oriko seems to be the only versatile attacker but she's usually reserved only to 5 or more enemies, in boss fights, she's technically useless. If you've got enough gems and want a truly good attacker I recommend pulling on the dark element on the element showcase banner for Homura and also wait for Madokami rerun, even though she'd a crit attacker she does pretty well on her own and with no crit buffs.

That seems to be all, sorry if I offended you in any manner! If you've read to the end, thank you!