How can I make a long wall perfectly level?
Use a 70 foot water level.
Here is a diagram of one.
Use 1/4 inch tubing or slightly larger.
Fill tubing with water before submerging one end.
The container of water is required because a huge difference in water level at the stick end, translates to a tiny difference in the water level at the container.
That keeps the actual water level almost the same, no matter how much you move the stick up or down.
NOTE: the accuracy suffers on windy days.