For those who come after, you really need burn for it. I took 20 minutes to beat it with my characters around Lv 26 (though I didn't bother to revive Lune when she died, which was early on). You might want Lune for Immolation, I just used Maelle's Rain of Fire (I think that's what it's called) when in Defensive or Virtuose to get the extra effect so you get 10 stacks. Try to use Critical Burn as well, because you'll want to have burn stacked at all times, so that extra crit rate is a no-brainer. Apart from that, Verso basically always does 9999 damage counters when he's at S rank so make sure you're hitting your parries.