I've beaten it a few times with different setups, but what worked best for me was:
Two Rikku, Tank, Setzer, Ace, and DV (or any unit that can increase dark resistance).
The difference between getting murdered and surviving was insuring that your Tank could absorb the physical damage moderately well (you have two Rikkus helping in this) and being able to survive dark AOE magic attacks. Throw on Melody of Life, Soluna band/ring, Black Mask, Angel Earrings, etc to stack that dark resistance on as many of your units as possible. If you have DV with his 70% dark resistance buff and you already have dark resistance on your unit, you'll take little to no damage against those attacks.
For the damage portion, Two Pod153's Mirage + Fire Element + Ace -75% Imperil + Setzer Double Dice finisher worked really well. I actually also had Pod153 on my second Rikku, and had her dual wield fire elemental weapon, and chained Mirage with DV on her spare turns (not necessary but its speeds things up). Usually I just chained with Ace (who also had Pod153 and fire weapons) With Ace's Imperil, you're able to do consistent amount of damage on all units.
You have to make sure you can do over 11% damage on the mini Marlboros per turn, as if they hit below 10% HP they self destruct and you have to start over again. Given the Marlboros are DEF/SPR breakable, make sure you have a debuffer can really smooth things along; DV can fill this role, or Rikku with her pouch can do it as well. Also insure you have someone who can dispel (or has bushido freedom) to remove buffs from Great Marlboro once he goes below 50% HP.
Only tricky part is getting all of them to die at the same time.