As long as she is fought alone, Dark Mindy is easy to defeat. Her regular attack and her Passado target only one character at a time (this last attack inflicts 15 hits), so as long as at least 2/3 of the party has the Auto-Phoenix ability, they are harmless.
When she is alone, or only with Sandy, her Overdrive is Mega-Graviton. It causes magical fractional damage at 43.75% of target's max HP, as well as several status ailments: Doom, Slow, Sleep (for 99 turns), Darkness (for 99 turns) and Silence (for 99 turns). Occasionally, Dark Mindy will also use Calamity to inflict Curse, Poison, Full Break, Darkness and Silence on one target.
When Mindy is fought alongside both Sandy and Cindy, they will use their deadly Delta Attack Overdrive executed only by Dark Cindy, which always deals 99,999 damage per hit (6 total) and ignores Auto-Life. Mindy has the least HP, but her Evasion is so high that even with 5x Aim + 5x Luck most attacks will miss.
The entire party would optimally be stat-maxed (except Magic), and if the player plans to use physical attacks, Luck must be very high. If the sisters are fought together Dark Mindy should be killed last as she takes the least number of attacks. Sandy should be killed first as she has less HP than Cindy.
Dark Mindy is the most evasive enemy in the game, and is the sole reason for the often-quoted required Luck of 230 for "effectively maximized"; 205 Luck is needed to never miss her, which increases to 220 if under Darkness, while 230 is needed to ensure critical hits without fail.
As with any other enemy, Dark Mindy can be killed instantly by Yojimbo'sZanmato attack, along with the other Dark Magus Sisters if the player fights more than one of them.