This achievement is unlocked during the 4th mission of the second Slavs campaign, "Boleslav I".
There will be 10 ships to destroy in total, but first things first:
You will start the map in the southwest corner, with villages all around you. The ships won't spawn until you take village E, so take that one last. In order to take village E, you will need a catapult, which you can either build yourself, or find an abandoned one next to village D. Once you've taken village E the ships will start spawning from the east side of the river. If you still have your catapult, great, but if not there'll be 3 available for you to man in village E. You won't need more than 2 catapults for all the ships. Take your catapults and station them at the northmost point of the riverbank. They should take care of the ships themselves with no input needed, but make sure you protect your catapults, as enemies will start coming from the west (you will be warned by a friendly cavalry squad).
Once you've taken out 7 ships the objective will complete, but keep your catapults where they are! The 3 remaining ships will spawn shortly, and once you take the final one out you will pop the achievement.