Killing an enemy while they are under the effect would be like Bucking Bronco while they are in the air, or on fire from Devil's Kiss for example. See the "Well Rounded" achievement description to find out more details about the Vigors.
You will add to this number in the course of getting trap kills, the 8 combinations, knocking enemies off columbia with undertow, and getting your falling kills. Zap and shoot is an easy method as from the first bioshock. Use Shock Jockey to zap them and then shoot them in the head for an easy kill.
Murder of Crows, Bucking Bronco, and Shock Jockey should help greatly at netting you this achievement with combination gunfire kills.