Hi there, thank you for playing!
Aqua only does physical damage, so depending on which Members you have currently and their levels, you can do one of these:
- Use Ina's Tako Protecc or Kiara's Kikkirikki to increase defense
- Ame's Watson concoction to deal some Bad Status in order to interrupt Aqua's attacks
- Gura's Hydrodinamic Shark rises physical evasion to 100% so she won't be hit for a couple of turns
- Kiara's Battle Stance gives her 100% Counter, so she will deflect any physical attack directed at her
Hope this can help you to overcome this boss!!