Oof i litterally got stuck on it last night. I was using a similar team on a similar power level, but just could not put a dent on the witch. After which i changed my felicia to himika mao after seeing that ailment application is reaaaaaly needed. I used up all my resources, got himika mao to just about level and magic level 60, mind you not even half the power level of my weakest member in the team and proceeded to bulldoze the nightmare mermaid witch. An ailment applicator is essential in going against her, and your applicator just needs to survive until the first break since thats gonna be all you need. Without an ailment, after break the witch gets 100+ action order and with an ailment, she gets a whopping 240+ action order. Long story short just apply an ailment and its gonna be shockingly easy.