Pretty simple. About halfway into the game you will need to head to a beach where your friends are located. Once there, head down onto the beach and there will be small crabs scattered around on the shore. If you go down there and they are not there, simply go back up the hill near the campsite then come back down to see if they spawned.
Now just kill one and the achievement is yours.
The campsite and beach is located in the "Shipwreck Beach" section of the map.