You can do this on the very first mission while you're in the fight club. Right at the corner of fighting ring (in the ring itself) in a bottle of Plagued Spirit you can pick up (it's amber colored with a dead rat in it). To unlock the achievement you need to throw it at a group of 3 people clustered moderately close together. One easy way to do this is to aggro them so they'll gather to fight you then toss the bottle at them. If the cloud gets three of them the achievement will unlock. Just save before hand so you can try again if you miss and/or you don't actually want them mad at you as you play through the game.