I killed lots of guards in smoke and it didn't pop cumulatively for me, so...
For this method, you will need:
a full Assassin meter with Arrow Storm available
Cherry Bombs
Smoke Bombs
There's a Landmark called Fatih Mosque in the Constantine North district (right next to the Fatih Camii tunnel entrance). It has lots of guards at each doorway and more inside. Map screens and video below.
Climb to the roof of the Mosque and look in at the middle, where the fountain is. Toss a Cherry Bomb NEAR the fountain (NOT IN the fountain, your bomb will not detonate in water). Many guards will investigate the spot of the bomb, so then toss a Smoke Bomb in their midst. Then activate your Arrow Storm by holding
.
Boom, achievement.
This is also where/how I got the Mosh Pit achievement