Another great place to do this that provides more than two enemies at once is in the last level of the first story.
When you reach the climactic fight with Barbossa, you will be tasked to hit him for one damage, which will cause him to run away and spawn zombified pirates. These pirates can only die in moonlight; therefore, you should run into the water where you can still fight and it is dark and attack them. 5 or 6 will be on you at once, and they respawn just as you do when you die. Keep killing these pirates, and you'll unlock it in no time as you have already gathered some kills in the course of the story.