Kill 5 guards while under the cover of a smoke screen bomb.
To make the bomb, you must have ingredients to create it. Any shell works technically, but impact is best. Use the British Gunpowder to ensure widest area of effect. The main ingredient to even make the bomb is Phosphorus. You can get these ingredients in many ways, looting dead bodies, chests, rewards, etc. You can even just go buy them if you can't manage to make them. You can press to see through the smoke after throwing it. This achievement only requires you to do this 5 times. You do not have to do it all at once, but you might end up doing that anyways while going for Lightning Strikes achievement.
Make sure you have cherry bombs in addition to smoke bombs. Use the tunnel to travel to Faith Camii and climb to the roofs around the courtyard. throw a cherry bomb to lure the guards (there's about 8 of them) to a spot. Then throw the smoke bomb and start the slaughter :)