There are a few ways to make enemies get knocked off Columbia (when they fall off the world and die). They must have health as they fall, you cannot kill them because dead bodies will not count towards the stats.
When an enemy is near a ledge, use Bucking Bronco to lift them up. Not shoot them a few times as they float to knock them back a little bit. They should be hovering over the sky with no ground below them. Just wait for the effects of Bucking Bronco to cease.
The absolute easiest way to knock them off is to press and push them back over railings or ledges using Undertow. Enemies that are on barges will be prime targets as there is not much distance from ground to ledge and you can push multiple enemies off at once.
Great places for these are at the End of chapter 28, Factory, where you help Liz into a vent, and 38, Hangar Deck,whilst fighting in an airship.