Kain and Rosa join your party just before this battle, and such a fact
becomes a big problem if you realise that they are probably weaker than
the rest of your current party. Concerning Rosa, for example, she may be
easily knocked out by a single of Barbariccia's attacks, which is an
obvious problem, as you may need to revive her quite often.
You should also be aware that this boss has two different forms. In the
first one, she is easily damaged but tends to counter with a powerful
electrical attack, while in the second one she turns into a hurricane-like
foe, which can only be stopped by hitting her with Kain's "Jump" move.
The best way to defeat her would be by casting magic, which would make it
impossible for her to counter your moves, but the problem is that you
currently have absolutely no characters that can use offensive magic.
As soon as the battle starts, have Rosa cast her "Slow" spell. Now, when
she is in her first form, you should have Rosa for your healing tasks
(with support from Cecil's healing and revival spells), have Kain always
use his "Jump" technique. As for Yang, have him use his "Power" command
multiple times, since it doubles the damage caused by your next physical
attack, allowing you to cause a lot of damage easily. In fact, it works
even better if this character is equipped with his "Hell Claw", which you
can gather in this very same dungeon, and which may randomly induce the
"Poison" status on the boss, which helps you cause some more damage
without much effort. Whatever you do, just DON'T strike the boss
physically with the other characters, or else she will counter with her
powerful electrical attack, which damages everyone.
When she turns to her second form, feel free to heal everyone (by then,
she only has a single powerful attack skill, the previously-mentioned
eletrical one) and stack more damage using Yang's "Power". Using Rosa's
"Pray" is a good idea, since it reposes some HP and also some MP. When
you're ready to proceed, simply have Kain use his "Jump" ability, and
apart from causing some damage he will also revert the enemy to her normal
form, mentioned above.
The biggest problem here, as you can probably notice, is the enemy's
powerful electrical spells, which heavily damages everyone. If they just
can't stand such an attack, feel free to head to the last dungeon and gain
a few more level ups in your characters, you'll be needing them in the
uture, and they may also end up being useful against this same boss.
ecco, te la tradurrei io ma devo fare alcune cose e sn impegnatissimo scusa...