BOSS ANALYSIS : Dark Ixion begins to throw some interesting things into the mix when you battle him. He has two main Attacks. The first is his usual Attack; it will do some physical damage and cause Confusion, Armor Break, and Mental Break, doubling the damage you take. Ouch. You can also see a different one as a counterattack; it’s the same, but causes Sleep, not Confusion. You can also see Aerospark (physical damage + Dispel (removes Auto-Life too)), Thundaja (non-elemental-Thundaga, really, to all - has a delay effect). Of course, it’s Overdrive - Thor’s Hammer - will come, too - it’s unblockable and non-elemental and also induces Power, Armor, Magic, and Mental Break. Ouch.
MAIN STRATEGY : If you plan on buffing any whatsoever, it pretty much has to be through armor Auto-abilities: key ones include Ribbon, Auto-Protect, and Auto-Haste. Auto-X-Potion (as I effectively call it) also works well here due to the myriad attacks flying around. When the battle opens, Shell would be a great thing to get on everyone to help deal with the prominently-powerful Thundaja - either cast it one by one, or simply go for Mighty G. Beyond that, you shouldn’t need much for this fight; if you’re having trouble when it comes to constant 10,000+ damage, though, try Mixing Ultra NulAll for the various stat boosts that come with it.
Anyhow, as for an offense? The faster this guy goes down, the better; nothing here is really worth being hit by, so Quick Hit is by far the recommendation. If you’re going to go for a magical offense (which is about 17% stronger), you should Doublecast Flare and Ultima a lot. But, basically, once you’ve halved the damage you take and immunized yourself from the ailments, Auto-Potion should be able to take care of the boss’s main offense (other than the Breaking Attack; you might have to Dispel to live through some attacks after that). Just remember to send in an Aeon to deal with Thor’s Hammer when it comes up so as to avoid a ton of Breaks - or that death, of course.
You will have to fight Dark Ixion twice, by the way. It’s clear how to do so, though. The Overdrive abuse tactic also works well.