This achievement is VERY picky. First, you have to get exactly seven synergies for this to work. Get any more or any less, and you're not going to get it. Second, the profile you're trying to get the achievement with has to be involved in all seven of the synergies. If it's not, then no achievement for you!
With that in mind, Mojo Synergies occur when two or more players in co-op activate mojo powers at once. The easiest method for this involves exploiting the song "Bombtrack" by Rage Against the Machine with the fan trick described in "Perfectionist." Put the vocals on Disciple, and play either drums or guitar on the highest difficulty you're comfortable with. Get to the point where you have enough to activate mojo, wait for the vocals to fill up, then activate it for both "players." Rinse and repeat six more times, and this should pop for you.