This achievement is story-related and cannot be missed.
While in the Bowa-Seko, after you escape from the prison you acquire a mission for a reprisal killing versus the UFLL or APR. This killing is dependent on the previous mission. After completing that mission you will do two "house cleaning" missions for 25 diamonds each. The first mission is acquired in Mosate Selao at either the UFLL HQ or APR HQ. Which headquarters is dependent, again, on how you've progressed through the story. Who you begin the mission with is trivial.
After completing that mission (you will have to kill two targets at the same location) you will receive a cell phone call from Rueben at the Marina bar. You will proceed there to receive the second mission of the house cleaning. Once you finish this second mission the achievement shall unlock.
Again, it is story related and impossible to miss.