Run the Gauntlet
Rather than giving a ton of effort in the early stages of your base you can wait to do the campaign until you have really strong units that are much above the level of the missions. The campaign missions do not increase in difficulty while your base and units increase in power. So once you attain a high level base and powerful units you can jump right into the campaign and crush the missions obtaining 3 stars every time.
I easily busted through the campaign in the Classical age, having 30 legionary (Regular infantry) 10 Sagittarius (Regular ranged infantry), and 4 Chariots (the unit that costs 5 housing space) available every battle. Remember that Chariots go straight for Towers or Ballista's. 3 star the first 20 missions and this achievement is yours.