As you progress and move through Medici you will occasional see a pale blue icon on the side of the screen with some text and the word : Encounter
(it will also have a specific number of metres e.g. 344m). Move towards it till you get there and follow the instructions given. Usually, it will say something like: Free the Rebels and Clear Heat or Pick up Agent and Kill Them by accident
. Once you've done the task then your achievement should pop up.