On the main menu select Skirmish
Change your Home town/Civilization to Spanish (seville) (The flag Icon next to your gamertag)
Select Deck Builder -> Beginner
On row IV remove one of the icons by clicking on it
Select IV: Industrial Age
Select the 24 Pikemen card to place it in your deck.
Click Save and then back
Change starting age to Post-Imperial Age
Start the game and enter the cheat code: Nova & Orion
Select the flag icon in the bottom right corner of the screen (next to the mini map)
Choose the Beginner Deck
On line IV Select the 24 Pikemen option
This will deliver 24 Pikemen to your Town center and will pop the achievement.
Enter cheat code speed always wins to speed this up a bit.
Note: This has to be done using the 24 Pikemen card, not a combination of cards.