On the last level you will go into a free building mode.
Press A to build your first village and get infinite ground power. Now spam ground for a bit so you got room to build more villages.
Once you had a lot of ground and the trees are pretty much spreading all over then build 3 other villages.
Now if you haven't activated the mini cut scene then just keep spamming ground all over until you fill most of the map. This will take about 5-20 minutes depending on how many times you use the bonus (Y button).
Eventually you'll get a cut scene and the world will start to shake. Press A anywhere on the map and get your ground power going to fill in the ground that is falling down. You're villagers will get to the Sanctuary and build it. Cheevo <3