This is an achievement for taking the secret escape route. This unlocks when you shoot all 18 of the inflatable crocodiles hidden throughout the island. You can work on this after you eliminate all of your targets so you don't have to worry about anything else. I recommend using an elite guard's uniform (guys in red that roam around the upper half of the map) to make this as painless as possible.
I recommend following the video below (credit to Martinoz) while using my locations, as they're sort of scattered around despite some of them being bunched into one area.
The locations are as follows:
#1-7: 4 are outside the Snorkel Instructor’s hut, 1 hidden below the table, 2 inside his hut.
#8-9: 2 at the main swimming pool.
#10-11: 2 at the Villa’s swimming pool.
#12-15: 4 in total are outside the Restaurant on the beach.
#16-17: 2 outside Doctor’s Hut.
#18: 1 outside the abandoned hut, only with cleaning staff inside.
The exit appears at the south end of the island by the fisherman (follow the in-game compass).