Far Cry Classic

You will get this during the 20th and final mission "Volcano." Killing him is surprisingly easy. Even on Hard he only takes a few shots to die. Getting to him is a bit harder.

Right at the start of the level, look left and go through the door into a small room with tons of weapons/ammo. Be sure to get a rocket launcher a sniper rifle and an AR. Exit and enter the next room. There will be around 20 or so guards in this room, many have shields. One guy has rockets, he is the #1 priority, once you see him take him out ASAP. The rocket launcher makes quick work of the shield guys as does laying down and unloading AR rounds into their legs.

Once this room is clear and you've picked up armor/ammo, precede through the next set of doors. A cut scene will trigger and after you'll be placed behind a big desk. Krieger will be directly in front of you and will move to your left. If you're fast enough 2 sniper shots should do the trick. Spraying him with an AR will also kill him pretty fast. On Hard you might need to peek out to the left and take out the soldiers then shoot Krieger. Don't use the rocket launcher to kill him, Valerie is near him and can easily be killed which will make you have to restart the checkpoint behind the desk.

Once Krieger hunches over and puts his head down, duck down and wait for 2-3 seconds, a cut scene will trigger and the achievement will unlock!