Danger! There is a 1/3 chance of it attacking with Bad Breath. To avoid being wiped out, equip armor that guards against status effects and attack with fire. Kimahri can learn Bad Breath.Scan description
When fought inside the Tower of the Dead area inside Sin, the Omega Ruins, and the Monster Arena, the Great Malboro always begins the battle by ambushing the party and unleashing Bad Breath, which inflicts several status effects: Silence (100% chance for 3 turns), Darkness (100% chance for 3 turns), Slow (30% chance for 3 turns), Poison (150% chance), Confusion (80% chance), and Berserk (30% chance). Aside from this, Great Malboros have a 33% chance to use Bad Breath, unless a party member is Poisoned.
The Great Malboro will use a Gastric Juice attack on a single party member, and after falling below 50% HP, can use a Full Auto volley of gastric juice hitting all characters and a strong Munch attack on a single party member; it also counters Delay Attack and Delay Buster with Munch. Against aeons, it will only use Gastric Juice, and when Provoked, it will use Full Auto.
It is a good idea to equip active party members with armor that resists some of the status effects from Bad Breath, with Confuseproof being vital. Having one active member equipped with a First Strike weapon will make the fight easier, as they can use Provoke before the Great Malboro's first turn to prevent Bad Breath, switch to Yuna to summon, or simply use Flee. They can also switch to a reserve member with access to those abilities, namely Tidus. Having someone doublecast a Fire-elemental spell is also effective, as Great Malboro is weak to Fire. Yuna can summon an aeon, which are immune to status ailments, and Valefor can delay the Great Malboro indefinitely with Sonic Wings.