Instructions
Cast all of your debuffing spells on Barthandelus as soon as the fight begins. The most important spells are Deprotect, Deshell and Slow, which get rid of his barriers and slow down his attacks.
Cast buffing spells on your party, such as Haste, Protect and Shell. Barthandelus has very powerful attacks, so you need to keep these spells on everyone to avoid getting knocked out.
Have two players attack Barthandelus with their most powerful magic spells while one player casts all of the preparatory spells. This will whittle away at his HP while you prepare for the main assault.
Launch your assault as soon as all of the players have cast spells. Barthandelus staggers quite easily, so you will be able to take out large amounts of his HP if you can overwhelm him with your attacks. Heal your party before lunging into an assault, and then keep attacking while Barthandelus is staggered until your HP is low or he comes out of staggered status.
Watch your screen for two specific attacks: Dazega and Apoptosis. Barthandelus has a full range of spells and abilities, but these two are particularly dangerous. Dazega causes about 1,500 damage to everyone in the party, so you need to heal immediately after he casts it. Apoptosis removes all positive statuses from your party, such as Haste and Protect, and it removes all negative statuses from Barthandelus, such as Slow.
Recast all your buff and debuff spells immediately after Barthandelus casts Apoptosis. This spell gives him a huge advantage over you, so you need to keep yourself protected while decreasing his stats.
Cast a Summon if Barthandelus uses Doom on your party's leader. The battle is timed -- although there is no actual timer -- and if you do not beat Barthandelus fast enough, he will cast Doom to end the battle quickly. If you cast one of your Summons, the Doom timer over your leader's head will stop until the Summon is over. You can continue attacking Barthandelus while the Summon animation is happening, so use the opportunity to launch your final assault. If you do not kill Barthandelus before the timer counts down to zero, you will automatically lose the battle and have to start over.