The two solutions made before mine provide the essential information for what to do to obtain this achievement, my solution is just to point out some of the glitches my partner and I encountered while doing the Sons of Samedi missions and what to do if confronted by them.
-After completing a mission, host gets a black screen: This is kinda frustrating. If the host gets a black screen, that means neither of you gets an autosave and you'll have to replay whatever mission you received black screen on. Can't really get around this as far as I know, and after waiting for 5 minutes without any result, it's safe to say that the black screen will not go away.
-"Riot Control" mission doesn't complete because Shaundi is invisible and the counter for number of boxes loaded gets stuck at 11/12: This glitch is mission specific! If you're not at the mission I'm talking about, chances are you don't even understand what I mean. However, if you do encounter this glitch and are looking for help, here's the place to be. Right before Pierce brings the truck back and the cutscene gets triggered, make sure you have Shaundi in the back of the warehouse away from the front entrance where the truck will park. Kill her within 5 seconds of the cutscene activating (the beeping noise indicating the arrival of Pierce will get fast), and she should be visible after the cutscene and the counter will hit 12/12 and you will be able to continue the mission.