This achievement is unlocked at the start of the mission "Hunted". Shortly after starting the mission you will enter a building where you can see the farmer being interrogated by some enemies.
Watch them for a short while and after Price says "Let's top these b*stards before they kill the old man." Do not kill them before the voice prompt, it didn't pop for me and I had to restart checkpoint. You want to kill the guy in the middle before he pulls his gun then kill the others. The farmer will run inside the house behind him and the achievement should pop.