There are 3 enemies on a little island in the south of the map (see map). These 3 enemies are really well hidden and the most gamers are easily overlooked these enemies.
For the achievement you have to kill all enemies, all tanks, all half-tracks, all reinforcements and also the 3 scientist, don't knock out these guys!The achievement unlocks after you kill your last enemy!
You don't need to leave the mission, just search your last enemies if the achievement not unlocking.
At the end you have to killed over 200+ enemies in total.