The Top Ten Most Challenging Xbox 360 Achievements of All Time (So Far)

Khamsa -- 05: Deus Ex: Human Revolution "Pacifist" (100 G)

Required Action: Complete Deus Ex: Human Revolution without anyone dying by your hand. (Boss fights don't count.)

Perhaps the most annoying of the tough Achievements due to the fact that it is possible to kill without knowing that you killed during the bits where you have to shoot at a vehicle to disable it -- remember you can't take a bullet back, right mate?

This achievement starts from the very first moments of the game -- when Jensen has yet to be augmented, so killing anyone in the Prologue locks it out -- and while you can kill the boss mobs (they do not count for this because you HAVE to kill them) you can NOT kill any of the soldiers that may be present during the boss battle.

What makes this one tough is that if you end up directly killing -- or if any of your actions causes the death of an NPC by accident -- unless you are aware that you did that and stop playing, reloading a previous save, there is nothing to indicate in the game that you have invalidated the requirements for this -- so you can easily end up playing through the entire game only to fail to unlock it at the end. Sigh.