But if you lack flashbangs and you are on PC, I believe the default key is "F"... funny thing is that if you are not quick and totally behind the enemy, he may turn around and spray you full of lead. I know, it happened to me yesterday
Flashbang as mentioned, and they can even work in tough situations where you have 2 or more guards in a tower that raise detection warnings as you're creeping up the stairs.
(Skip to the 8:03 mark for the flashbang sneak.)
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