FFIII did not give me any trouble there. But then again. I power lvl! So here, I scrounged this up from Gamefaqs!

16~. Dorga (22800 HP)

Hum, he's back... not only his physical attack damage is quite big, but so
is the damage caused by his magical attacks. Exactly what we were supposed
to see in this kind of character, I guess...
Anyway, he seems not to have any particular weaknesses in terms of
elemental magic, so your best bet is probably attacking him with physical
strikes. You surely have to include the usual White Mage on your party,
but you can also put 3 physical attackers in there, if you want. Your
whitey should do the same task as in battle before (i.e. heal your team in
EVERY turn) and the other members should be constantly attacking the enemy
with physical strikes.
A good idea could probably be the one of putting 2 Knights and a Monk in
the party, or any physical strikers you like, but it turns out being fully
up to you, just be sure to take good care of the White Mage, as you'd be
pretty much in trouble, without her.
At level 41, my characters were more than enough to beat this foe, but you
will then have to fight yet another boss battle, exactly after this one...



17~. Unne (21800 HP)

She seems stronger than Dorga, at least in my opinion. Like what happened
to the boss you fight just before her (look above ^ ), she has no weakness
in terms of elemental magic but she is weaker against physical attacks.
For obvious reasons, you will be using the very same party that you used
in the battle before, and your strategy should be almost the same. Your
enemy goes mostly for magical attacks, with a powerful spell that puts ANY
of your friends with less than 10 HP (however, this attack will never kill
anyone, think about it as this game's version of a powerful "Demi" spell)
and other powerful spells. You can also use some attacking spells, but I
don't think it is a very good idea.
Mostly, try to stick to physical attacks and healing all your party EVERY
turn. In case more than one of your members dies (except for the White
Mage, which you should revive as soon as possible), try keeping your
strategy up and attacking as usual, but DON'T forget that HEALING is the
most important thing in this battle. Without it, you will be really f*cked
up, and there's not even much you can do to save you from defeat.

Thanks and credit go to Mykas0