This is a non-missable story achievement. Once you kill 13 of the most wanted targets you will get this. You get in between 1-4 kills per mission and they are usually the main objective or yellow diamond during flying missions.
I personally got this at the end of Operation Safe Harbor, but since it is based on the main hub it is random.
The achievement won't pop until you are back at the main base, but don't worry it will pop on your way to story completion.