Instructions
Attack the airborne Gyarados as it flies in at the end of the Deep Miracle Sea level.
Unleash electric attacks offered by the Pokemon Pikachu and Shinxor or strong attacks such as Grass (offered by Treecko and Chikorita, among others) to bring the creature down quickly. Avoid using ground-based attacks, as these will have little effect.
Take the Phione Dew as your reward from the Phione for defeating the Gyarados.