When fought inside the Omega Ruins, Great Malboro will always begin every battle by ambushing the party and unleashing Bad Breath. After the first turn, each of Malboro's three main attacks: a gastric juice attack on a single character; a volley of gastric juice hitting all characters, and Bad Breath, all have an equal chance of being used. Bad Breath can inflict status effects like Poison, Confuse, Darkness, and Berserk. It counters delay attacks with a chew attack, which causes heavy damage to the character that delayed it.
It is a good idea to equip active party members with armor that resists some of the status effects from Bad Breath. Having one active member equipped with a First Strike weapon will make the fight easier. If that character has Delay Buster, they can even defeat the Malboro before it takes its first turn.
Having someone with a First Strike weapon who knows Provoke is useful; if provoked before it makes its turn, instead of using Bad Breath, it will use a weak attack that hits the entire party for less than 1000 damage, giving the player plenty of time to finish it off before its next turn. Having a member doublecast a Fire-elemental spell is also effective, as Great Malboro is weak to Fire. If Yuna has First Strike, she can summon an aeon (which are immune to status ailments) before it takes its first turn. A player merely trying to traverse the Omega Ruins can also give First Strike to any character who knows Flee.
Another option is to use Valefor who can delay him indefinitely without suffering the counterattack with Sonic Wings.