In act 1, head to the sheriff's office, investigate the table and go downstairs. Free Merok.
In act 2, when you have the choice to explore from the Oak Tree, head to the western site (Brotherhood Encampment) to meet Merok again.
Later on in the chapter, you will have to choose between attacking the enemy camp (by the Brotherhood) or go to the bridge. For this achievement, attack the camp. you'll meet Merok for the third time. Agree to his plan and the achievement will pop.