I think it's actually even harder in the next update. I had to nerf them a bit. Using immortality, false promise, a second-chance bracer, Davy's Blessing, Auto-Life, and anything like that feels required.
I also got Dante and Bessie to completely absorb elemental damage, while Ray had auto-reflect. This makes Luna a lot easier to deal with especially when she reflects spells onto herself and deals half her HP in damage.
The hardest part of the battle is Francesca using time warp, or "certain hit" abilities that pierce false promise.
RPGMaker in general used to have a memory leak. The engine is notorious for a reason, and the Android version isn't official; it's enabled with a fan-made package. I imagine it could perform very inconsistently on various different smartphone systems. I couldn't even get the music to consistently work with it, meaning a massive part of the game's identity is lost in that version.
I apologize for any issues you have with that version; unfortunately, it's not something I can help with. In general, I say you should treat the Android version as a last resort method to play the game.