To obtain this achievement, you must defeat two Karen enemies. These are the flying enemies that start to appear around Mission 5 (or thereabouts), and spew clouds of pepper spray on the ground.
For this achievement, simply kill one with the Tailormade gun (SMG-style sewing machine) and then kill another using your belt whip (melee attack). These kills can be done at any time across multiple levels, so don't worry about it too much.
If you still don't have this achievement by the end of the game, you can find a Karen enemy toward the beginning of Mission 6.