I just found the ultimate way to have 20 only when you found 1 pirate lord.
1- Found one pirate lord, save the game next to it.
2-Reload the game by continue, Shoot him and when you see the kill on the left side of the screen (The ship still there) quite save.
3-Reload and repeat. Has the game count the kill while saving but the save always bring you back few second before. You get the kill and have always a new pirate lord at the same place.
