To unlock this achievement, you will need to finish the entire game on the Agent difficulty. Before choosing your difficulty, you can choose your health system. There are 2 to choose from, one being Modern and the other being Classic. On the Modern health system, your health will regenerate over time after taking damage, whereas on the Classic health system, it will not and you will need to find armor and health packs to regain any lost health.
To make this achievement as easy as possible, choose the Modern health system and then choose the Agent difficulty and then proceed to complete all of the missions to unlock this achievement.