Aqua Sonic: PP 369, EP: 36. Ranged strike at a row of three.
This will be hard to dogde, but easy to survive. Don't let her hit Atsuma twice with it.
High return song: EP: 84 revives allies at half health. Uhh...What allies? Am I missing something?
Cure song: EP 18, Heals 198. You should be doing at least 5 times this much each turn.
Clear voice: EP: 30. Removes status condition. If she wastes a turn on this, it's a win for you.
As always when fighting a water-type, Atsuma is a fragile heavy damage-dealer. Keep him protected! It'll also help to have someone healing him nonstop. There's no need to use EX's in this fight.