Simply progress through the campaign on any difficulty.
You will unlock this achievement once you finish the "Strike!" mission in the "Kingskeep" chapter.
Kingskeep
- Night Fight
Royal Arena
- Ranger Management
- Melee Mastery
- Lord of the Arena
Dark Forest
- Direwolves!
- Vikings!
- Jammed...
Kingskeep
- Defending the Crown
- Keeping the Crown
- Ballista Oversheep
- Strike!