I did it by upgrading the Bard once on Aire and using the mouse for healing. Brandt as Bandit with the bow Arbor sold and the Attack song hit for over 100 damage per attack. With the protection song I was able to withstand Arbaroc's assault no problem.
I was lvl 19. white mage and bandit. their is a fairy cape in the tree dungeon. put that on the bandit, the wind cape from dungeon on the white mage. When the boss takes to the air don't attack it, he seemed to counter more when it was in the air. Just use that time to heal and boost. And just have the white mage heal every turn of battle
My strategery: Brandt as Salve-maker, Aire as White Mage. Aire spent her turns healing, and Brandt fed Dispensary'd Ethers to Torte for constant Blizzaras. When I needed to replenish Dispensary, I prepared a Spell Focus to one-two a Blizzard/Blizzara.
lol torte went in naked on this one for me and got one-shotted at the start, hero was salve-maker and aire was a black mage.. just cycled dispense and healthcare on hero + hi-potion and focus+aero for aire..
I just beat him. I had one healer with all of the white magic possible: cure, cura, raise, shell, protect, and healthgiver.
The other two were equipped with red shots, one being a bandit, though i don't see that being all that important.
The strategy is to have the two attackers attack and cure alternatively. I think the mouse is good for curing mostly, as both his attack and blizzara barely do damage. While the bird is on the ground, the two attackers keep their own hp up by alternatively attacking and curing. The white mage should be casting shell 3 times and then protect 3 times, unless the bird leaves the ground. It may take a couple of tries getting it right, but from what i've seen, the bird lifts off after 3 or 4 turns. I recommend going shell->shell->healthgiver->cure. Oddly enough, my healer shelled the one who needed it most last, the mouse, who takes a lot more damage than the others.
When the bird is in the air, focus on healing. At least until the bird is about to die. At some point, he starts to go crazy when he's on the ground, but his strategy is the same in the air. At this point, you should reverse your strategy, buffing when hes in the air and curing when hes on the ground. His Leaf-aga or whatever does ALOT of damage to all your party, and he does it twice and fast. Finish him quickly now because his berserkiness means hes almost dead.