Simply progress through the campaign on any difficulty.
You will unlock this achievement once you finish the "Final Breakthrough" mission in the "Donkey Island" chapter.
I would suggest using Javelin, Javelin Burst & Potion of Conversion for crowd control.
And Apple Grenade & Tri-Stone for attacking the enemy castle.
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!
Donkey Island
- Flag of the Island
- Donkey Transport
- Enemy Outpost
- Call of Privy
- Final Breakthrough