I have a guide for this achievement on my YouTube channel.
The goal here is to destroy all of the pinecones Hugo throws into the water. This is during the sling tutorial in chapter 1, part 1. You'll want to constantly be aiming your sling and holding the fire button to keep the next rock spinning so you can release it quickly when the pinecones go into the water. After a 2nd kid joins the game Hugo will start throwing pinecones a bit faster. Keep destroying them as fast as you can.
Once Amicia tells the new kid to deliver the coup de grâce he'll throw a large rock in the water. Any pinecones still in the stream at this point are missed. When Amicia says this line if you know you can't destroy the last of them quickly pause the game and choose to reload the section. This will restart you right by the log ready to try again. If you don't do this before the cut scene begins you will have to quit out to the main menu and begin chapter 1 all over again. Attempting to restart section after the cutscene will just put you right back after the sling training.
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