Create a scenario in which you place a very large number of buildings. In order to guarantee the game would count it I only used larger generic buildings such as a blacksmith and monastery, although you could experiment with smaller buildings.
You will need to spend a considerable amount of time placing your buildings in rows across the entire map. You will need thousands.
Then make some gaps in your buildings, as fair and even across and down as you can.
Place siege rams or whatever unit you wish to give to the AI. Remember Mongols get increased ram speed.
Make sure the AI is set to the correct position in the players tab.
Remember you can flip the players around if you want to go to for the 10000 buildings razed achievement.
Depending on how many buildings you have placed, you may need to repeat playing this map on "single player" under the saved scenario several or a dozen times.
You can press your window key to do another activity on your computer as the AI razes your buildings to sweet dust.
Although it may not make a difference, setting your enemy AI to hardest may improve razing speed.
Remember you will need a sustainable building to prevent an instant game over. A castle works well.