You'll find a fish in a number of areas. I did this in the club. You can do this in the mission in the club ("Club Bam Bam") or just walking around. First shoo out the fish display. A fish will fall out on the floor, grab it, and find anyone and attack the enemy til they are dead.
You may want to weak up a normal enemy with a few attacks so it only takes one or two hits from the fish if you're having problems with this.
I believe I did this against a normal NPC just walking around however, I don't know if you are required to kill an enemy with it.