Have a guard incapacitate 3 civilians while he's poisoned.
While it's not required, using the Gold Bomb will guarantee you get this achievement easily.Craft one and make sure you also have poison darts. You can also try buying them from the Black Market Dealers. Go into a crowded area, usually a market works well. Find some guards and throw the Gold Bomb at their feet. The crowd will rush violently towards the gold and the guards will react shocked and may come after you or fend off the crowd. Quickly shoot one with an arrow and they start swinging away as they are dying. You can throw a second bomb if you want if it looks like the crowd is dispersing, but they will be fighting the guards so you should get it with one.
An alternate method would be to poison a guard with a dart, then throw money at him (not the bomb). This will cause people to rush him and you won't be detected as a threat by other guards. You also won't have to use any bombs at all.
the best way to do this is to throw money on the ground near some guards so there is a group of people near them and then shoot three guards with a poison dart so then the others dont notice and start to attack you. i did it first try
I got this unintentionally during the mission where I had to rescue two assassins. There were two guards guarding an arch, which people were walking under, and I poisoned both the guards with poison darts, and they started swinging around there spears, hitting civilians in the process, much to my amusement.