Total War: THREE KINGDOMS

Trebuchets are probably the easiest (maybe the only?) way of getting this achievement. They're only available to strategist characters, but it is a siege weapon, so you can immediately start battles against walled cities with it. As soon as you're in the battle, just select all your trebuchets and target whichever wall you wish. It'll take a minute or two, depending on how many trebuchets you have and how good their accuracy is. They should outrange the city's arrow towers and gate fortress, and easy/normal AI is pretty loathe to leave the city (not sure about harder AIs), so you don't have to worry about defense. Regular shot is probably better than flaming shot against walls as it has better damage and walls can't be lit on fire. Once the wall is broken, you'll get the achievement after the battle's over.