I'm running Kirika, Alina, cool Homura, pluvia Madoka and Himika and I'm still getting my shit kicked in by level 30 tower, the Mermaid witch. How do I improve my party? Everyone is lvl 100 and magic level is mostly 120.

Also I hope someone will explain me what is the difference between "debuff" and "ailment". Ailments look like debuffs except they're called... ailments, not debuffs?