First, to make things easier on yourself wait to do this achievement until after you have taken over the city (without doing the epilogue of course). What this does is make it so members of the Saints are everywhere, and are likely close by to lend support during your stops. This also means that you won't have to put up with the enemy gang coming after you other than those chasing you in vehicles. Also remember you can't bring any backup with you to the mission. As soon as you hit Y to start the mission whoever you have with you will be dismissed automatically. I'm not sure if your backup carries to the next mission if you simply start the next after you finish one, since I didn't use any up through level 5, and after that I stopped and saved after each one, which I do recommend you do too.
The Carnales dealer is a stoner who drives a painted up hippie van. You can fit two others into the van with you, but they won't be able to do anything but ride along since there are no windows for their seats, and getting out of the vehicle unless you have to to get a fresh one or to revive the dealer is a waste of precious seconds. With this dealer you shouldn't need any extra backup really since all that really comes after you are the Carnales vehicles and no Police unless you shoot the cops a lot, even in level 8. For this one really just stay in the van and shoot the Carnales in the vehicles and you should be fine. This is the easiest dealer of the three.
The next two dealers are a different animal altogether. I would recommend finishing the Chinese hitman and the Redlight Escort first to get Mr. Wong and Samantha as homies. Wong has a SMG and she has the AK and they're both tough. You might Consider doing the Rollerz dealer Laura (in the Suburbs) as she also becomes a homie after her eight deals are done and she carries a shotgun. As I mentioned I finished the first five levels of each of these with only my player as support. Whether you do that or not I would suggest the following, ESPECIALLY at or after level 5: As soon as you are pulling up to the first stop in the mission, bring up the pause menu, go to your phone and call either Mr. Wong or Samantha (Will or Wheel Woman, who you should have unlocked already can also help if you can't get the other two, or Laura if you've finished there). The other two, Chicken Ned and Zombie Lin, are armed with melee weapons and can't help you while driving. I would try to alternate between them for each mission. These two dealers are hard in the later missions and if you save between them as I said earlier you can't bring homies into the mission from the start, so whoever you have with you will get dismissed. Sometimes the homies are finicky and they may tell you no, so try to get at least one of them. In any case, you should also try to get somebody from your gang to join you at the stop. You can add two others to your group, so a homie plus a gang member OR two gang members in the odd event you can't get either of your gun-armed homies to join you helps a lot during the mission. As others have noted in level 8 of these two dealers you will have the gang chasing after you along with the FBI who will ram and shoot you like mad, so be prepared to switch vehicles about halfway through the mission. Having the homies with you will be a big help there too.
*Oct 22, 2012 For the first time this solution has gotten a negative vote. Sure wish I knew why since it seemed to be doing well before that.