You have to kill Brock, Suzi and Chad.
You will randomly get them as final kills. The narrator should say exactly these people: Final Brock, Final Suzi and Final Chad, and if you killed all of them, you will get the achievement.
Just play the game and hopefully they will appear as soon as they can.
By the way it took me about 13 hours to get it, so it's full random as I said before.
Good luck!