You will need to create a bomb that spreads poison for this achievement. 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 Datura Powder. 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.
Focus on crafting both a Cherry Bomb and a Datura Bomb (the poison bomb's name). Find an area with a lot of guards and throw a Cherry Bomb in the center. You may also use other forms of distraction based bombs as well. Of course, there are other ways but the ones listed will make it easier.