Taking out a guard swiftly, cleanly and most important... Stealt
When in the dark and can't exactly get the guard where you want him get close but still in the dark and press L2 to whistle. The guard replies sometimes like "What the" or "What the Hell". He walks to the place in which he heard the whistle and turns and stops then starts walking. Stay crouched and go up behind him fast and hit R1 before he gets out of the shadows and it will hit him in the back of the head making him unconscious so that you can carry his body into the shadows to dump it.
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