Reminder: I tried to use as less Action Cards as possible so you can follow the steps easily. But if you have good Action Cards, you can use them if you want, as long as it doesn’t disturb the instructions I gave below.
Bounty Bloom Deck
La Signora has super high HP so it's recommended to bring along a healer so your character can keep up with the damage. That's why I bring Barbara; her Burst heals so much you don't have to worry about health at all. In addition, her Skill also helps in cleansing the Cryo/Pryo effect your active character has, which helps at certain points.
The last enemy with surprisingly straightforward kits, but with annoying Special Rule and RIDICULOUSLY high HP. So La Signora has a passive skill to inflict Sheer Cold or Blazing Heat (depends on her stat - whether she's in Cryo or Pyro mode, respectively) using her Skill. With the Special Rule, your last active character before ending the round is bound to be afflicted with Sheer Cold or Blazing Heat, which makes it easy for Signora to trigger reactions.
First of all, start with Nilou as the starting character. Make sure you obtain at least 2-3 Hydro Dice and 2-3 Dendro Dice and the Katheryne Action Card. If not, restart the challenge until you get the dice I mentioned. Follow the steps below after that:
Nilou's Skill > Use Katheryne Action Card > Switch to Nahida > Nahida's Skill > End the Round before the opponent
Switch to Barbara > Barbara's Skill > Barbara's Basic ATK > Switch to Nahida > End the Round
Nahida's Skill > Switch to Nilou > Nilou's Skill > End the Round before the opponent
Nilou's Burst > Switch to Barbara > Barbara's Skill > End the Round before the opponent
Switch to Nahida > Nahida's Burst > Switch to Barbara > Barbara's Burst > End the Round
Barbara's Skill > Nilou's Skill > End the Round