All you need for this is a decent sized convoy (6+ Liners or Wargalleons) fully loaded with sailors and weapons. Sail around pressing on each town until you find one that isn’t currently defended by a convoy. Once the battle starts move your ships into position (marked on the map) and take out the town’s defence fortresses. Once the fortresses are down, attack the marketplace and the town and achievement are yours.
This will be unlocked if you play through the Adventurer campaign as it’s the last mission.