So I spent three hours building up the stats of my characters, and killed enough baddies to get the almost-100-million gold to buy one of the soul shards. So I went and tried to kill Rondo again.
So I bumped the levels of my characters up some more, and swapped Viola out for Falsetto (as Falsetto can built the multiplier up way faster). With Jazz and Allegretto both on level 66, and Falsetto on 58, I tried again. And died. Again. Gah!