If one dies, they will cast Arise, reviving the other dead Purobolos. This can make the battle long and tedious for an unprepared party. They use Self-Destruct in low HP, forgoing Arise. Purobolos are weak against Silence.
Another method is to take advantage of their Self-Destruct, as that stops them from casting Arise. Characters with physical-based jobs should attack each one separately until their HP is low, minimizing the damage from Self-Destruct.
Yet another potential method is to have a White Mage cast Silence on a Purobolos before killing it. This will prevent it from being casting Arise, allowing them to be taken out with ease.
In the event of a worst-case scenario (i.e. a completely unprepared party), it is possible to constantly kill them until they run out of MP for Arise.
The Blue Mage and a Beastmaster make this battle simpler; Control can be used to force them to Self-Destruct, and Missile is an excellent way to damage them without killing them. Magic Hammer drains their MP so they cannot cast Arise.
A Bard makes this battle a breeze, as using Romeo's Ballad every turn can end the battle without the Purobolos taking action.