All you need is a little aim, and a lot of luck. Place your drop right on the tip of the arrow and release. About every 6 throws Gepyo will eat a drop. You can do this 3 times before he swims away. To get your prize, do one battle, then come back to the pond. You will get a rare drop.
Mana Drop
Look at the left side of the wood tree screen. The flying Ecomon will sometimes carry a clear Mana Drop. Select it and the Mana Drop will fall. The sound of a Mana Drop being collected will also be heard.