You will reach a point near the end of the mission where your brother hands you a cellphone, and splits from you. There will be two enemies here and your goal is to steal the truck.
Enter the poppy field with the area, and as soon as you see the prompt, press up on the dpad to distract them. As they approach the sound, go to the table near the middle, steal the handgun, line up both enemies in a row, steady your aim, and take the shot. It doesn't need to be a headshot either, as long as the bullet lands high, it should kill them both. If you miss or get spotted, reload the checkpoint and try again.
Possible on any difficulty, but recruit is probably recommended to make it as easy as possible.