You can find a Scorpitron, a large scorpion-like machine, roaming around the Colorado wastes while traveling in your vehicle. You'll be given the option to attack it directly or use your sneak ability to grab some gear without it noticing. Eventually, using sneak too many times will lead you into a battle anyway as you'll get caught.
While it may seem a daunting fight at first, it can be 1-Hit Killed by ramming it with your vehicle. Simply select your truck in battle and move it all the way to where the Scorpitron is and it'll be killed instantly for an easy 10G.
EDIT: It seems as though you MUST take down the scorpitron in an overworld encounter and will not unlock during the "War of the Worlds?" quest.
Another opportunity to grab this achievement comes from the "War of the Worlds?" quest, where you'll get the opportunity to fight a Scorpitron alongside some friendly robots in the Tellurium Mines.