The "island of the dead" is on mission 7, Serpent Siege. After you've crashed your helicopter and fought off enemies as its being fixed, you will be allowed to fly again and your new objective will be to destroy AA turrets with airstrike markers.
At this point, you can follow the river on the right until it becomes an ocean and then just keep flying straight and slightly to the right, out over the water. You will pass a small island with a crate on it, and then come to a larger island where you see some friendlies (green triangles) shooting at zombies on either side of the little camp.
Just destroy all of the zombies for the achievement. Some of them take A LOT of damage, like 10+ missiles, so just keep spamming at them until the achievement pops.