Killed 5 enemies while they are falling.

There are multiple ways of earning this throughout the game, such as shooting enemies as they jump off a barge or skyrail, or begin to fall as the Bucking Bronco effect ends. However, the easiest method can be performed as soon as you obtain the Undertow vigor in Chapter 27, The Factory.

Once you have learned Undertow you need to equip it and the Shock Jockey vigor at the same time. With the two vigors equipped, search for some regular enemiesno heavy-hitter or armored typesto get the 5 falling kills on. After locating some simply use Shock Jockey on them, immediately followed by an Undertow.

This vigor combination will electrocute the enemy and simulaneously send them flying. The water+electricity effects will amplify the damage exponentially, allowing you to kill most regular enemy types with a single try while they are flying (falling) through air.