Have the Highland expansion activated, and play using a "Local Player" in addition to your main character. Immediately go into the Highlands, and run around there, keeping the second character always within 6 feet of your main character, and behind them. Eventually you'll pull a card called "Tumbling Boulder". Rolling anything but a 1 makes the boulder roll 6 spaces behind you, and into any other character that stands there. Since you have your second character behind you, they'll lose a life and you'll unlock the achievement.
If you accidentally get the Tumbling Boulder card with the other character, immediately exit the game and reenter, then roll your movement again. You might have to do this a few times to have them land on a space where they don't draw a card. Then you can move your main character to a space that does draw a card, and you'll pull the Tumbling Boulder, because the Adventure Card deck doesn't shuffle on exiting and reentering.