Another easy place to get this is on the second island, if you activate the side quest "fathers burden", the third guy you have to kill is stood in a group of 5, fire an rpg or cook a grenade and the achievement is yours
Edit: You can also get the achievement for earning $1500 player poker during this quest. Just go all in everytime
Note while the method for the poker achievement has worked for a lot of people, a couple have pointed out that they have lost. I advise making a save on a usb stick beforehand if you're worried about this.