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.