First off, choose "Reaver", and make sure to unlock the "Kick Hobblet" ability. This ability is located on the red tile on the left hand side of the board, and only costs 500 gold.
Once you have this ability for Reaver, go to the Hobbe Caves level, and proceed to the draw bridge. Here, just use the A button to kick one of the hobblet enemies, and immediately press X to shoot your gun still pointing in their direction. This will cause the enemies to be shot out of the air, and you will unlock this achievement.
If you miss, keep trying a few times, and if you get too far in the level, you can always just restart it.
Additional details can be viewed in the video: