Kill 13 enemies while riding ziplines.

You will find ziplines at a majority of the bandit camp locations throughout the game. The best way to unlock this achievement is to find a zipline in a bandit camp, then making a save and then use the zipline while killing enemies nearby, i found the Rocket Launcher and it's splash damage the most effective method, but be sure not to kill everyone in sight. Then reload your save and you will be at the entrance to the location you were just in and can repeat your zipline kills, repeating as much as you need until the achievement unlocks.