Lift an enemy using Bucking Bronco and blast them with the shotgun right as the vigor expires. The timing may take a few attempts to master, but with an upgraded shotgun and Bronco, I managed to take out a group of 4 in one fell swoop.
You can also use the Shock Jockey/Undertow combo. Shock a weaker enemy followed immediately by an Undertow. This should render them muerto and count as a single skeet shoot. Repeat x4.