I'm guessing you're in chapter 1, so here you go:
Boss: Guarding Beast
He's weak against all elemental magic, except gravity, so cast some -ra or -aga spells (fira, firaga, etc). Also, you can reduce his accuracy by having a songstress cast Darkness Dance. My advice would be to equip one person as a white mage to heal, another as a songstress to cast Darkness Dance, and another as a black mage to cast spells. It shouldn't be too hard, since he only has 2886 HP.
