Early in the game, you will reach the "Club Bam Bam" mission. Inside the club, you will fight several goons in a room lit with aquariums. Grapple one of the enemies, and drag them to an aquarium to initiate an environment kill with the aquarium. Once the aquarium is smashed, a fish will drop out. Pick up the fish using RB, and use a heavy attack to instantly kill an enemy with it and get the "A Slap In The Face" achievement. Note: If you happen to miss with the fish, you will need to grab another fish; there are two aquariums in this first area.