Begin the first boss battle by dashing out the range of his Arrowhead attack. Try to break his tail, so you can attack from behind with no fear of receiving damage. If you can break his arms, you may receive a Baraka, a great rune, but it is risky, as he may use Violent Purge. If you need to heal, use your Items. If you have Sylphide in your party, make sure she has Free Item equipped, and is the only one healing. Ballistic Rhino is quite slow though, so take advantage of this to run circles around him to recover AP. Keep attacking, and he should go down.