Hunters can found throughout the campaign and multiplayer. Simply kill 17. The best place to obtain this achievement is mission five (Expendable). As you progress through the level you will come up against 4-5 at once. You have to kill them in order to finish this part of the mission, the flamethrower is in a weapon crate as you enter the area. The flamethrower or incendiary rounds are the best way to take them down. They also show up in the multiplayer on game modes Survivors and Heroes, and killing them there will also count towards this achievement.