This is best done during either Drug Busts or against non-D.U.P. enemies, as this may take a few tries and you'll need at least three enemies to accomplish it. Once you unlock the Neon Beam, getting the upgrade that allows more time is extremely helpful for this, otherwise you'll have to be perfect on your aim as well as have the enemies very close to each other.
All you need to do for this is hit either their heads (Evil Karma) or leg (Good Karma), it doesn't matter if you mix between the two, but going for the head is a lot easier to pull off.