Just like herself in the anime, Kaoruko is a massive pain. Why? Confusion, utter boatloads of it. Her damage is nowhere near the levels of Maya, but the ceaseless confusion stymies many teams that would have worked otherwise. Her secondary Artifice, as if one wasn't enough, is her self-cleanse on CX, which throws wrenches into strategies relying on MaxHP% DoT.
Pied Piper of Hamelin Michiru is the support unit built to counter Kaoruko. With an inbuilt passive that blocks Kaoruko's Opening Act and a 2AP which continously keeps the confusion immunity up, she completely trivialises the fight. She's also very cute. Otherwise, you could consider the Confuse RES memoirs listed alongside. However, note that this will require restarting until Kaoruko fails to apply the Confusion with her Opening Act, and you'll still have to subsequently rely on resisting her Ending Act to give that particular unit some time without confusion. It is tedious and generally not worth it, especially if those memoirs are not MLBed.
Without her, a lot of praying and use of MaxHP% DoT will be needed.
3★ Passion is Contagious
Confuse RES (10%/25%)4★ Distant Renaissance 2019
Confuse RES (15%/30%)Strategies involving Pied Piper Michiru.
1) The reading is cancelled; wrap it up.
Other than Michiru, involve either one flower self-buffer or a full team. Proceed to delete Kaoruko.
2) A better DoT than depression.
Apply until Kaoruko is deceased. Refer to CR Mechanics for the list of MaxHP% burn/poison to use here, with Gawain and Diaochan being the most preferable.
1) One-hundred-and-one Home Runs, Thirty-three Fouls
Awakened Mahiru, can solo Kaoruko, if supported by some unit skills and some chipping from a flower team in front of her, due to her heal spamming. You do have to gamble with the confusion, which is why reducing Kaoruko's health as much as possible before the fodder gets wiped and boosting Mahiru's stats with unit skills are important
Mahiru's memoir should be determined by how much of Kaoruko's HP she needs to take off on her own and her level of tankiness. Calibrate accordingly to maximise success rate.