This is story related and cannot be missed.*** Spoiler - click to reveal ***In the final mission, 'Afterlife,' work your way through the level until you encounter Ammit. After enraging him (by shooting him in his mouth), he blows a hole in the wall. Enter this hole and be teleported to a battle with the evil witch queen Seteki. After defeating Seteki you will be teleported back to Ammit.
Repeat this once more. After you defeat Seteki a second time, she is grabbed by Ammit. Ammit tries to swallow her but she is hanging on for her life by his teeth inside of a yellow circle. You must make a choice here: shoot Seteki or not.
For this achievement, shoot Seteki, unlocking the achievement.During the cutscene that follows, press
and restart the checkpoint. You will need to go through the second battle with Seteki again, but when it comes time to make a choice, choose the opposite (in this case, do not shoot Seteki) to unlock "For King and Country."