At a certain point, events with Cecille and Sera will pause. This is your cue to , who will initiate a new chain of quests and unlock further developments.
Spend the night alone (without training others) to trigger unique night activities with NPCs in Mars' room. Hero Party Must Fall Guide
Before you can defeat them, you must understand what makes the Hero Party, well... heroes. At a certain point, events with Cecille and Sera will pause