This is a story based achivement and may seem to be missable but it actually isnt as you can replay the last mission once it has been completed with just the other option left.
On the last mission when you have to choose between saving Shaundie or going for Killbane go for Killbane. Once you have killed him you will get the access to your final mission. Play this mission to the end and kill Cyrus Temple (flying in his VTOL). The easiest way to kill him is to clear out the platform from rocketlouncher enemies so they cant annoy you. Then go to the crates and pick up a rocket louncher or like me the gatling gun thats lying there and finish him off. Now just jump off the ship by taking the VTOL thats spawning.
After the credits have been finished (you can also skip them) your achievement should unlock right at the loading screen.