Her special ability, Passado, is more dangerous than Cindy's or Sandy's special abilities, and she has the least HP. Once any of the sisters has been defeated they can no longer use their dreaded Delta Attack Overdrive. The Magus Sisters do not have any elemental affinities.
Using Paine's Full Throttle dressphere to spam Sword Dance and/or Fiers on Mindy first is a good way to start the battle. After defeating Mindy the party should go after Cindy, since she has healing magic.
If the player doesn't want to use special dresspheres, a party consisting of an Alchemist and two Dark Knights is one good setup. The Dark Knights should repeatedly use Darkness to damage all three sisters at once, and it won't take more than a couple of turns to defeat Mindy.
The Alchemist should mix a Potion and a Hi-Potion every turn to achieve a Mega-Potion. Stashing Mega-Potions also works, but Stash has a longer charge time than Mix. Mixing a Lunar Curtain and a Light Curtain may be a good idea, to grant Shell and Protect to the party at once.