Saints Row 2

Simple, just start from the first Samedi mission and work your way through them with a co-op partner present (it doesn't have to be the same partner for each mission). There aren't that many to do and as far as I am aware you needn't do every stronghold although some may require you to do before you can advance.

Just bring up your map, find a Samedi icon and go do it. This will also work towards the 'Remind Me of the Babe' achievement if you haven't already gotten it.

Use this website to locate all of the missions and strongholds:
Map no longer available, sorry.

I also had tons of trouble doing the mission 'Riot Control' Shaundi would always disappear whenever the truck came, i tried all the methods of killing her and stuff... Nothing worked! Until i decided to kill myself. I hosted the game, started the mission and when the timer came on warning of pierce coming got my partner to blow me away! Worked first time. If all else fails try this out. Remember too turn ON friendly fire :) Hope this helps some people out!

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.

Okay, I just did these, the achievement description is complete the Sons of Samedi missions in co-op, but it should read missions and strongholds, since the final Sons of Samedi mission can't be unlocked without first completing the strongholds (the strongholds also have a checkbox at the bottom for co-op, similar to the missions). 12 Sons of Samedi missions, 4 Sons of Samedi strongholds.