This boss is pretty epic, but he isn't too hard. Doctor's code and potions
can be a godsend in this battle if you don't want to switch to medic too
often.

Anyway at first this battle is a lot like the havoc skyship fight in that
you must take out the other targets besides the main one first. Good buffs
the SYN can use are Shell, Haste, Bravery , and Faith (of course this
depends on which characters you're using). Fang can use Slow and Curse,
while Vanille can use DeProtect and DeShell, but debuffs won't work right
away.

Start by targetting one of the 4 other targets besides Barthandelus. Use
Libra first since each target is weak against it's own specific element.
Then attack with RAV/RAV/COM. You can use a MED to heal or use potions.
Do this until the 4 targets are dead and then the real fun begins.

Barthandelus should now be vulnerable, so cast your debuffs, and your
buffs while using MED or potions to keep up your health. Attack him
normally until you see him beginning to use Destructo. When you see this
come up IMMEDIATELY SWITCH TO RAV/RAV/COM AND ATTACK HIM AS MUCH AS
POSSIBLE. If he gets this move off too soon it may be game over right
then and there. If you attack him enough times, he will "flinch" and will
then continue charging the attack. When you see him flinch, heal yourself
as much as you can. The attack should now be much weaker but it can still
kill you if your health is too low.

If you can manage to make him flinch everytime he uses Destructo and
survive the second wave, then you pretty much got this fight won. Keep
up your health, keep fighting, and if he starts destructo again, drop
what you're doing and blast him till he flinches.

If you're still having problems, then use a fortisol and aegisol before
entering the room.