Just to expand on the strategy that Mayadome mentioned: once Bicocettes (those small, purple, shielded enemies) are introduced in the later levels, you can stand in a safe zone next to the fence just before the third waterfall (not including the two behind the two wooden bridges), where the fiends won't hit you, and kill every enemy with Volley ammo when a Bicocette walks past - quickly leave the safe zone to pick up the Volley ammo from the treasure chest the Bicocette leaves behind and return to the safe zone, and then repeat until the last minute or so, where you then need to rush to the beach using any Volley and Death ammo to quickly kill anything in your way.
Here's a picture of where the safe zone is, with a Bicocette behind Yuna, for reference:
http://imgur.com/q0fiuQCEnemies will just walk past you here without damaging Yuna, so this strategy is very easy and relatively safe.