As you are playing you will eventually be able to "sabotage" an enemy when you get it DO NOT USE IT IMMEDIATELY. After the battle repair your ship. then look at your map and find another pirate not near a sentry tower. sail over to them as you approach the "keys" on the bottom of the screen will change to the battle keys. As soon as the other ship appears and you have the skull key on the screen hit that key and if you haven't been shot you will immediately destroy the opponent and pop the achievement. Hope this helps. I will try to answer any questions that you might have in the comments.