Doing it legitimately, without boosting, then the mode Manhunt is by far your best bet. One out of every four matches, you're bound to end up on a team with at least one teammate who's so kill-happy he'll ignorantly steal your Poisoned targets.
If actually applying the Poison is where your problem is, then it might just require some practice. You need to be very close for the Poison to work and with lag being what it is in this game, it might not even always work. A good trick for Brotherhood is to run up to a target who is in the stunning animation (knocking out one of your teammates): he won't be able to defend himself while in the animation and you will be able to get a "free" Poison off on him.
"You know, it's all very sweet, stealing from the rich, selling to the poor."
playing (primary) far cry 3 playing (secondary) tes v: skyrim watching the walking dead