From jman12343 on gamefaqs
Basically, I tried 3-3 about 15 times killing them from only tree branches and several times from other tree positions, and never got the trophy.
My solution that just worked for me was immediately after doing a run through on 3-3 and killing single enemies from only tree branches, I ran all the way to the south exit to New Orleans (as soon as free roam loaded up again). Stay in the Bayou, don't travel or bring up the menu. Quick edit: Doing 3-3 may or may not be necessary, but I say do it jusr ro be completely safe.
I went north to the fort, and on the way killed all 3 guards from trees. Once there, lure the guards out front of the fort to a tree branch right up a hill immediately south of the smuggler. Kill them from that branch. Go into the fort and get as many of the guards in there as you can to chase you. I was able to lure 3 more, 1 from the center, and 2 from the back wall that jumped down. Then go out the side door and lure those 3 as well. Go to the same tree branch. After killing about 3 of the guards from this group, I got the trophy.
Keep in mind to kill every single guard separately and from a tree. Double kills not only don't seem to count, but they also get rid of 2 potential guards. If any guards cluster up and won't move, go down and just run into them to push them around and go back up. I also did all 15 kills directly in a row this way, so any other form of kills may reset the number.
Now, I'm not saying this is a guarantee since nearly nothing in this game seems consistent, but maybe this will help some of you out there. Sorry for the rather lengthy description, but I wanted to give every detail just in case it was all necessary.