This Seeking Tech video guide will show you how to get the Waterfall Chaser trophy and achievement in Crisis Core: Final Fantasy VII Reunion.
This will unlock when you do the following:
Collected ten chests coming down the waterfall in the hills of Gongaga.
To trigger this hidden mini-game in Chapter 9, when you need to “head to the hills to follow the winged figure.” After the cutscene is completed, run around in the same area to fight several random battles.
When you cross the river at the bottom of the waterfall, you should then trigger the mini-game. If the mini-game doesn’t appear, don’t leave the area. Instead, fight more random battles before returning to the river.
As for the actual mini-game, you should stay in the middle unless you see the treasure chest dropping to the left or right. Use the following to easily unlock this trophy: