It's time for another big boss fight! If you remember the Havoc Skytank, Barthandelus puts up a similar fight, because he also has four secondary targets. You have a lot of choices for your battle setup. Barthandelus's main body is immune to debuffs at the beginning of the fight, though after destroying the secondary targets he loses this immunity. A Sentinel isn't really vital in this fight, though it may help for desperation situations (within the Combat Clinic paradigm, for example). Barthandelus's attacks are mostly Magic, so take off any Black Belts you're using and replace them with Rune Bracelets in preparation. Personally, I prefer to use a lot of Synergist Abilities during this fight, since they always work and they're a bit easier to keep up, but there are many ways to achieve victory. Defeating Barthandelus unlocks Crystarium stage 7.