Age of Empires II: Definitive Edition (PC) - Achievement - Smoke on the Water
Purchase 20 Dragon Ships in the Historical Battle "Lake Poyang (1363)"
This level can be quite tedious. You start with a navy and soon encounter a village that will join you providing you with Villagers, a Trebuchet and a few Chu Ko Nu to add to the Chu Ko Nu you start with in a Transport Ship. The idea is that you slowly push using your different forces which clear the path for the next force: Use the Trebuchet to destroy towers, Chu Ko Nu to kill land units and your navy to destroy enemy boats. A timer is visible which shows when a friendly transport ship will arrive and deliver supplies to a temple that is being built. You must protect the Transport Ship otherwise it's game over. This is a repeating theme throughout this mission. Once you get to where the temple is being built, the village will join you, providing you with a more permanent base than the initial village that joins you, as well as a larger army.
In the very North of the map, you will encounter the 'Chinese Peasants' faction who will state that they make the finest ships in the land - These are the Dragon Ships that you need for this achievement. All you need to do is pay them 100 gold via the diplomacy menu and they will provide you with a Dragon Ship. Do this 20 times and the achievement will be yours. It appears there is perhaps a delay or maybe it does not register all the ships, but to be safe, just keep buying them until the achievement triggers.
There is no shortage of resources and there are also a couple of islands that contain gold that start occupied by enemy forces, so it's advisable that you clear these if needed. The Chinese Peasants also have a village on the far right of the map which contains a Market, so you can send Trade Carts to them to obtain gold that way. There is also a Relic in the village which will generate gold for you once you bring it back to your Monastery (you don't start with one and will have to build one). The achievement triggers before the end of the level, but I would also advise to save regularly as it can be easy to lose a Transport Ship.
NOTE: Achievement triggers at around 41:15 in the video - The remainder is just completing the level