If you are a very thorough explorer, you should get this with little issue. Most abandoned buildings have a handful of raiders inside. You will get quite a few toward the end of the main quest when going up against the Enclave as well. Killing slavers is always acceptable as well.
To mop up the remains of the achievement, simply save your game and go on a killing spree. Take out entire towns until you get it, then simply reload your previous save.
Note for PC Users: Instead of bothering with bad karma, use the console command "player.placeatme 0002F6C4 X" to spawn a generic raider, with "X" being the number of enemies you want to spawn at the same time. This depends entirely on how good your computer is. If you try something ridiculous like 200, you're going to crash your game. I wouldn't do more than 15-20 at a time unless you have a really good machine.
If you use the code "tgm" you will enable God Mode, allowing you to wipe out these large groups with no effort (unlimited health and ammo). You can also use the code "player.additem 00004322" to give yourself the Alien Blaster weapon which fires very quickly with a high damage output.