In case you didn’t read the The Aeons’ Locations section earlier, when you encounter the Dark Magus Sisters on Mushroom Rock Road, there will be a brief scene and "minigame" beforehand. Basically, if you can outrun the summoners chasing you, you get to fight the three Magi one by one rather than altogether. It’s a HUUUGE benefit for this battle, as you can tell if you’ve ever used the Magus Sisters. I’ll end up assuming that you are fighting each one by one, since it’s a simple-to-win game.
OVERALL BATTLE PREP : Yeah, with three varied enemies to fight, you’ll going to need a decent set of skills. Basically, it all comes down to the armor for this section; your stats ought to be 250~255 by now, or you probably wouldn’t even want to bother. Ribbon is very key to preventing the myriad ailments you will see throughout the battle; Auto-Potion with an X-Potion-only stock to heal; and Auto-Protect and Auto-Shell to halve a lot of the damage so that Auto-Potion can take effect at under 9,999 HP to basically nullify damage. Since most attacks here are physical, it may be wise to swap Auto-Shell for Auto-Haste because of the high Agility you’ll see here (mostly Mindy), but that’s up to you - it also helps stop Slow effects from the Mega-Graviton everyone has.
DARK SANDY ANALYSIS : Dark Sandy isn’t a whole lot to be surprised at, especially if you’ve fought Dark Anima already. There’s a basic physical Attack, which does some damage and will Petrify. You’ll also see her signature Razzia, which is damage plus the removal of Auto-Life. >_
Mega-Graviton will only be used if the lone sister’s Overdrive is full at least. If the three sisters are together and have full Overdrives,then there’s trouble - Delta Attack. This will be six non-elemental, non-reducable attacks to everyone; it also has a Dispel effect and can remove Auto-Life, meaning that you have to send in Aeon cannon fodder when the three sisters are at max Overdrives or you basically lose.
MAIN DARK SANDY STRATEGY : Assuming you’re running Ribbon/Auto-Haste/Auto-Protect/Auto-Potion armor, you won’t have much to worry about here when you begin the fight. Mighty G for Shell might be useful if you don’t expect to kill her before her Overdrive fills up, though that’s likely better used just before rather than do something that might be a total waste (especially if you already have Auto-Shell, too). But, basically, once you’ve got those up, it’s Quick Hit a bunch of times for the most part. You’ll have to use Auto-Life a while after Mega-Graviton hits (a Doom counter of 1, to avoid its removal by Razzia), or just send in Aeon fodder beforehand (the recommendation).
DARK MINDY ANALYSIS : Not much to be surprised at from Dark Mindy, either. There’s Passado, which is an instant-KO to a single character via unblockable physical damage; the KO effect is blockable, though. There’s also Calamity, which is Silence, Darkness, Poison, Curse, and all four Breaks; Ribbon protects from three of these, but the rest are either "deal with it" or "Holy Water/Dispel". Like Sandy, she can use Mega-Graviton or Delta Attack if needed.
MAIN DARK MINDY STRATEGY : A great day to have Ribbon armor, no? That will get rid of most of the threat from Dark Mindy, anyhow. The Breaks are inconvenient when you’re hit by Calamity, though, so you’ll be Dispelling every few turns. Other than that, (Auto-)Protect ought to make Passado survivable and then nulled by Auto-Potion, so long as you’re not in Armor Break. Calamity, you use holy Water or live with Curse, and regardless use Dispel so you don’t die. And Mega-Graviton and Delta Attack are Aeon fodder attacks you can predict.
And your offense is predictable, too - Quick Hits, with whatever healing is needed. Doublecasting Flare/Ultima might be preferable, though, since they are ~42% stronger than a physical offense; that part’s up to you.
DARK CINDY ANALYSIS : And, surprise, Dark Cindy won’t vary much from her normal un-Dark counterpart. There’s a basic attack, which happens have a Dispel side-effect that removes Auto-Life. She can also use her signature Camisade, which hits your HP and MP, so you’ll probably use (Mega)Elixirs a few times in this fight. Other than that, she has the same Mega-Graviton and Delta Attack as everyone else.
MAIN DARK CINDY STRATEGY : Same general thought pattern should continue here. Auto-Protect and Auto-Haste work well here since Dark Cindy cannot get rid of them and they halve most of the damage you’ll deal; the only threat here, really, is Camisade’s Osmose. Quick Hit or - much preferred - Doublecasting Flare or Ultima. (With 223 Defense and 105 Magic Defense, magic is generally 2.12x stronger.) It’s not complicated; it’s just that Dark Cindy is bulky. Attack, heal your MP every now and again, and send in cannon fodder for Overdrives.