Eternal Sonata

This will unlock after defeating Rondo in the 11th floor of the secret dungeon Mysterious Unison. To unlock the dungeon, you must beat the 2nd last boss in the game in the Double Reed tower. Go back from there. Leave the Double Reed Tower and go back to the desert (Noise Dunes). Once there continue going to either the left or right (Keep going in the same direction), eventually (Approx. 3 times through) you will enter a different area with a door. If you are having trouble beating her, use the dungeon's many enemies to level yourself up a few levels then come back (I did it around 65, but some say you should be at least 80+). My preferred party was having Allegretto and either Jazz or Frederic attacking, with Polka healing(Equipping her with something that gives off light certainly helps). If you are in the dungeon and don't have any idea where to go, I would suggest consulting a map via Gamefaqs, which is what I used as the dungeon itself is very confusing without a map.

I just managed to beat her on my third try. I thought I'd provide the details of how I did it in hopes it might help someone out. My party consisted of Viola: lvl 78, Falsetto: lvl 77, and Jazz lvl 78.

I had Jazz and Falsetto attacking her non-stop. Falsetto would usually have no problem building up a Harmony chain in one turn. I had Viola stand off in the back using nothing but Full Barrage, normally getting off two volleys on a turn. A friend of mine told me to set my items to nothing but Angel Trumpets. That was the main thing that helped me survive. Since the trumpets only take 2 points to go into the items set, I had 25 revives. I never bothered with healing magic or potions. Just all out attack using these tactics.

Rondo died in less than 5 minutes.