While in the Hills of Gongaga, after you start the scene with Genesis and Hollander, fight up to seven battles in the hills, then try to leave the area. Zack will notice a chest falling down the waterfall. Select the "I bet there will be more" option, and play the treasure chest mini-game. Monsters will come down as well as chests. The boxes and monsters will come down four times, repeating the same pattern:

Wave 1 (chest): Middle
Wave 2 (monsters): Left and right
Wave 3 (chest): Far left
Wave 4 (chest): Far right

These waves will repeat four times after the fourth wave. The more boxes you get, the better the prize. If you touch the monsters, Zack will become temporarily paralyzed. The best way to dodge them is to get out of the river, then go back in once they are gone.