Mindjack

Do this on scene select as campaign can be a bit glitchy and some not count

If boosting with a friend have them join your game spawn then quit back out then rejoin, rinse and repeat till the achievement pops

If no booster is available pick a level from scene select and wait every now and then people will join from time to time (assuming the online isnt dead already)

You can either have a friend(s) keep joining and leaving.

What I did was play through the game with the game open so people can join and when it got full kick a few of them or one, so someone else will join or even the same person.

If after doing that and you've not unlocked it, you're best to just get to a spot in a level were no one who joins can progress any futher. And they'll just leave. You'll be surpriced how many people jump in and out over the night if you leave your game open like that.