To unlock this achievement, you'll need to finish the
Hearts and Minds side mission.
The side mission itself is built up of three smaller missions:
- Retrieve the King's Horn
- Retrieve the Savior's Amulet
- Retrieve the Champion's Bow
*** Spoiler - click to reveal ***
You can start the first one: 'Retrieve the King's Horn' in The Hidden City region, when you talk to Uchu in the Skull Cave. You need to progress in the main story until the mission marker will lead you there.
Completing it will unlock the next mission: 'Retrieve the Savior's Amulet' When finished, you need to progress the main story again.
After returning from the 'Mission of San Juan' region, you can talk to Uchu to activate the final mission: 'Retrieve the Champion's Bow'
Follow the marker to a new area, kill the Nahual (about three charged arrows on normal difficulty), then return to the cave and talk to Uchu one last time. This will trigger a cutscene, and the achievement will unlock after you regain control.