This is a story related achievement and cannot be missed.
Once you have landed with the VTOL you will get the quest to destroy some AA guns wich are troublesome for your reinforcements. The first one will be in plain sight if you follow the street: the second one can be found in a little outpost to your right if you follow the street. The last one can be found in the middle of the harbor. If you don´t want to use your ammo on them justthrow some explosive barrels close to them and use 1 shot of your pistol (they will all be destroyed after two barrels). Just be careful cause i always got a helicopter to go after me when i destroyed one of those AA guns.