Age of Empires II: Definitive Edition

A quick and easy way to earn these civilization achievements is to set up a regicide game with a treaty, and you can earn each Civilization achievement in just about 5 minutes each. You can repeat this with every civilization and every difficulty level. Again: Quick and Easy.

Civilization: : Any you need for the achievement
Game Mode: Regicide
Location: Coastal
Map Size: Tiny
AI Difficulty: Moderate
Resources: Infinite
Population: 50
Game Speed: Fast
Reveal Map: All Visible
Starting Age: Post-Imperial
Treaty: 5 Minutes

As soon as the game starts, build a second castle using all villagers.

Select you first castle and set the Rally Point directly next to the castle of your opponent. You should see the flags indicating that the king is barricaded in there. If he's not there, check the town center. Once the second castle is built, set its rally point to the near the same spot.

In castle 1, create 15 petards. In castle 2, create whatever your special unit is.

When there's around 20 seconds left in the Treaty just move your view to the rally points you set. You should have plenty of units next to the opponent's castle ready for war the moment the treaty ends. Immediately attack it with the petards. Be ready to switch to the other units to chase down the king.

Alternatively, instead of using your special Units. you can just use all Petards and receive:

I have been informed that this solution is no longer working.

This is actually very easy.

From the main menu, click on Learn to Play which will bring you into the tutorial. (I believe on your first play through of the tutorial you will not be able to change the difficulty.) No worries, it only takes about 3 minutes to point and click through the first part.
If you have not won Easy Going and Celtic Victory, you will also earn those achievements on the first tutorial play through.

Once you have the option to see all of the "missions" of the section of the tutorial, ie. 1. Marching and Fighting, etc. (Back out to main menu after beating first mission of tutorial.) Go to the bottom on the screen and change the AI difficulty from Standard to Moderate.

Replay the first mission Marching and Fighting, and in 3 minutes once you complete it, pop, you have the achievement!

This way is by far the fastest and easiest way to earn this achievement.