Easiest level to obtain this on is level 3 "the hunt for answers" after you follow the messenger and go up the elevator you will have a cutscene take place in front of the ruined home. From here make your way to the next level in front of the large white building that you need to sneak into.
On the left hand side there is an archer on the roof teleport onto it and stealth kill him from the right side this will cause his body to fall off the roof. below him are 2 guards holding a door, 2 walking back and forth and 1 in the back watching your direction. One the body drops down in front of them the alarm will go off and they will start to come closer to the body.
Unlike explosive death this does need to be done in one smoke bomb. Wait for the large red symbol to arrive above their heads, smoke does have a large area of effect.
Hint - you can stun 2 guards with your first smoke to allow more time for other guards to get closer.
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