First, complete Sir Gavin's campaign to unlock Luna's campaign.
See here for my guide to chapter 1:Solution for Global Greening in CastleStorm II
Your first task in chapter 2 is to obtain spirit essence. If you built a level 3 Spirit Drill in chapter 1, you will already have enough : simply visit the green field a second time to skip this objective.
Otherwise, attack the bandits defending the marked green barrels. After killing them, move onto the barrels to pick them up. Then return to the green field.
Next, head to the Duelling Grounds and pick a side between Kranion (NorthWest) and Spines (SouthEast). Visit your ally's castle first: you will recruit them and have a second commander. Then move towards your enemy's castle, clearing your lands as you go, and conquer them. If they move close to one of your castles, fortify it for an easier defence.
After you've taken the enemy castle, return to the duelling grounds for a hero fight. Charge Luna as far right as possible, to take out a wave of skeletons and archers. Wave 2 is a solo skeleton, so either retreat and prep your shotgun, or ready a 4x power attack to say hello.
Walk North to the gate to the human lands, which ends the chapter and pops the achievement.