I found an alternative which will help you get this time-consuming achievement in no less than 5 minutes.
How to Obtain Mercenary Army Achievement Easy
Get The Achievement
In order to get the achievement, you must first unlock the survival mode.
This is how you do it:
- Step 1: Start a new survival game.
- Step 2: Assign 4 builders and build a tavern.
- Step 3: Meanwhile, go to your store and trade the wood and materials you currently have for gold, it will give you 600+ gold and the possibility to purchase both mercenaries.
- Step 4: When the tavern is built, purchase both mercenaries.
- Step 5: Go to menu and restart the process from step 1 until you get the achievement.