Map 21 features two cloud pillars on each side of the map and a dynamic wooden plank in the middle. The mice a spawn in the middle of the plank and the cheese is on the right end of the plank. The shaman spawns on the right side of the plank.[confirm]
Don’t run to the cheese immediately, because this will make the plank slip off and all mice will die. Wait for the shaman to attach it and finally connect the plank to the hole. It’s important to not act hasty on this map.
Be fast: Attach a red-nailed small plank on the center of the wooden plank before the mice make it fall over. After the board is safe, prepare a yellow-nailed long plank and attach it to the right end of the plank in order to prepare the connection to the hole. Finally, attach a red-anchored long plank to the c-anchored one, which will both stabilize the construction and make the mice be able to enter the hole.
Another method, which is very unsafe and not recommended, is to use a cannonball after attaching the board.