This will pop at the same time as the following achievement:
These two achievements will pop after the cutscenes after you beat Count Waltz and the Calamity Wilhm at the end of Chapter 6.
I used Allegretto, Jazz & Frederic, however, I do recommend switching Frederic out for Viola. You get 280,000 XP for your three fighting characters, and 140,000 XP for your non-fighting characters at the end of this fight, so take that into consideration.
I had the Crimson Brooch attached to Allegretto, which doubles his attack and halves his defense. To counteract the defense loss, I had Allegretto holding one of our two pairs of Recovery Gloves, which heals 10% HP at the start of each turn.
For Jazz, I had him holding Recovery Orb, which heals 20% HP at the start of each turn. At this point, Jazz should be around Level 43, which means he'll have between 18,000-21,000 HP, which means he'll be healing 3,600-4,200 HP at the start of each turn. I also had Jazz holding the Werewolf Choker, which doubles the echoes you earn from him, which means with every swing of his sword, you're getting four echoes. Simply, with six swings of his sword, you can use the Harmony Chains, and that's assuming your echoes are at zero!
With these items equipped to the two of them, it doesn't really matter who your third character is - this battle is incredibly easy!