Killed 20 enemies by knocking them off Columbia.

There are a number of areas to easily farm this trophy.
One of the easiest is toward the end of the game during the Command Deck chapter. Vox troops will disembark their airships roughy 3-4 at a time on the upper platforms. Rush up to them or use the skyline. A combination of Bucking Bronco and Undertow vigors will net you several of these types of kills in each attempt. You can also use the same combination on the main deck to launch stragglers over the railing while waiting for the next airship waves. Elizabeth will keep you well stocked with salts, but there are a few in the boxes near the Siphon should you run low.