That's one of the many fights you're not expected to win. Just survive 6 turns and you will be given the option to follow Cindy's plan. Sometimes losing can give you more interesting results than winning xD
You can just spam Jeanne's shield, Lianna's and Mao's blinding skills and Cindy's healing to survive as long as you can. There will always be a certain amount of RNG involved, but just surviving 6 turns isn't that difficult.