Among other places to get this trophy, Sequence 3 Mission 3 seems to be good. After going through a few waypoints and cutscenes, you have to take out some guards. When this checkpoint starts, there is a fallen tree serving as a ramp just ahead of you on the right. Free run up and forward until you get to the last tree just over the guards. Try to spot the guy in white (should be toward the end, and below you), because he's more powerful.
Select the berserk poison, target the guard in white, hold Triangle, then release. Once in a while, he'll go the wrong way and get stuck. When this happens, restart the checkpoint. Otherwise, he'll go attack the other guards, and you'll get the trophy when five of them are dead.