UNLOCK: Capture at least one of every fiend in Kilika.
BOSS ANALYSIS : Every time on the first turn, you will be ambushed; unless you have First Strike and 35+ Agility, you will be hit. Granted, you REALLY ought to have more than 35+ Agility, and Auron’s Masamune has First Strike. Anyhow, expect Putrid Breath from this - it hits you with some damage, Darkness, Silence, Slow, Breserk, Poison, and Confusion, all at once. Expect to have a LOT of trouble if you don’t have Ribbon. Anyhow, there’s also Chow Time (physical damage to one), Gastric Juice Blast (physical damage to all, may Petrify), and Mega Gastic Juice (damage to one plus Power/Armor/Magic/Mental Break).
MAIN STRATEGY : How does one survive the first turn? You need either two things. It is highly recommend having Ribbon on your armor. Otherwise, Confuseproof, Berserkproof, and Auto-Med also suffice … but that’s a lot of slots, no? Alternatively, you can have 35+ Agility and First Strike on your weapon, AND keep Yuna out of the opening party - switch to her on the first turn and bring out an Aeon for some cannon fodder. It also helps you keep your Ribbon/Auto-Potion/Auto-Haste/Auto-Protect armor and avoid damage.
After that, it’s pretty simple - damage and heal. Putrid Breath is pretty unpredictable, so if you don’t have any of the two set-ups noted above, you need to fight with Aeons or quickly obliterate the boss with Overdrives, somehow. Ifrit or the Magus Sisters are recommended (since this boss is weak to Fire, and the Magus Sisters are strong). It’s simple once you’re past the first turn, really.