If you use broadside cannons over the course of the game you will end up shooting down stray torpedoes randomly so it's recommended to finish this achievement last, if it hasn't dinged by then. If you're some torpedoes short. Find a deserters mission which spawns leech rocket platforms and sit stationary, manually shooting down torpedoes. Use pulse turrets + ordinance tracker for this. Don't use flak turrets! The game has issues tracking this achievement so there might be a discrepancy between game state and the steam achievement.