Cats of Yesterday is the second stage in Never Not Summer!.
Due to the lack of support, Selena will have a hard time advancing as long as you never stop meatshielding. If you do just that, your ranged attackers (Dragon Cat, Gross Cat, long-ranged Uber Rares, etc.) are bound to kill her given enough time. Once Selena is defeated, Sunny Celeboodle will be an easy kill.