This is another Editor-achievement which you can get pretty fast without much effort.
Create a map, go through all the steps (you probably already did this while going for the "Maker of Labyrinths" which has been explained perfectly by the user "StK I") and now click on "waves". This is where you created a few waves of enemies. Within every wave, you're able to add a certain amount of monsters. These monsters will spawn in that particular wave.
Now, make sure the values of each group of monsters within a wave is pretty high, so its above 10.000 for all waves. This value can be seen in a green frame which appears after you placed a group of monsters in a wave.
If you're not sure how much "strength" you already have (which is in this case the monster-value you placed), you can go track the achievement to see your progess.
Don't forget to save the Editor-Map properly and the achievement will be yours.