Most of the way through the Dusk Island stage you'll get on a raft. You need to dodge the rocks in the ocean as well as the fish that will try shooting you or running into you. What I did was stay near the right side, as the fish seem to come from behind you. You can see their shadows in the water. From the right side I would shoot back toward the left side of the screen when I saw fish coming. The fish died in one hit so it was easy. I would suggest doing this with the blue mage, his spread shot regular attack makes hitting the fish much easier.