You may or may not earn this achievement through natural progression of the story; if not, load up Act 1 Episode 6.
You'll start near some dynamite. Pick this up and proceed through the entrance. There's a room adjacent to the entrance you just entered with a door to the south. Inside, there are three enemies playing cards. Enter this room and throw your dynamite. You may not kill all three of them, if not just let yourself get killed from the on-rushing enemies to the south and reload the episode.
It shouldn't take you longer than 30 minutes to farm all 100 kills using this method.
Also, you should break a few of the bottles situated in this room, earning progression of the below achievements: