Earn all Skills in one category.
There are three Skill trees: Seeker (blue), Scavenger (green), and Warrior (red). You can purchase these skills at any Base Camp and earn XP by doing literally anything in the game.
Some skills require completing Challenge Tombs while others are automatically unlocked via specific points in the story.
To earn this trophy you only need to fill one colour on the Skill tree. It may be wise to only buy crucial Skills, and then save the rest of your points up as much as possible to go back and complete a portion of the tree.