It helps if the entire party has Accuracy, Agility, Strength, and Defense at 255, and Luck at 80 or higher. As long as she is fought alone, Cindy is easy to defeat. If the party has the Auto-Phoenix armor ability, they will not cause much trouble.
Another strategy when fighting her alone is to have at least two characters with Auto-Phoenix and Ribbon, make sure all three characters have high stats, and are all hitting for 99,999 damage. Dark Cindy has no Evasion, so regular attacks are guaranteed to hit. Auto-Phoenix will keep all members alive, seeing as Dark Cindy's regular attacks target a single character, and Ribbon will protect two characters from the status ailments of Mega-Graviton, allowing them to heal the unprotected character. Auto-Haste is not necessary, but will allow more turns, resulting in a quicker win.
As with any other enemy, Dark Cindy can be killed instantly by Yojimbo'sZanmato attack, along with the other Dark Magus Sisters if the player fights more than one of them.