This is a complete achievement walkthrough of The Fall on Xbox One. During this playthrough I unlock every achievement except for "Merciful" which is unlocked at the very end of the game. I explain how to get that achievement near the end.
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40:50This achievement is for killing the final boss. After defeating him, he will be on the ground of the same room. Simply walk up and shoot him before leaving.
It's possible this achievement may be glitchy as the opposite achievement, Merciful, is. If this is the case and you unlocked the Merciful achievement during your first playthrough, before starting your second delete your save from the main menu and quit the game. This worked to unlock the Merciful achievement for me.