Na'vi 2nd zone
The first few missions have you investigating different locations and freeing prisoners while killing RDA soldiers on the way. When freeing prisoners watch out for gun turrets; they have a fast fire rate if they see you. Sneak around and try to take them out with your bow.
The trophy will pop once you have found the harmonies at the willow. This is a type of mission you will see frequently in your Na'vi campaign. To do this listen out for vibrations to find the harmonies.