Well I think the easiest way would be to go rent a bobcat and remove the excess dirt. My "out of the box" idea would be to stabilize the frame of your climbing wall with rebar that has been driven into the ground rather than allowing gravity to stabilize the structure. However, I like the idea of removing the dirt and starting with a flat surface better. Either way you have some problems. If you remove the dirt then there will be a gap between the ground and the fence and if you use the rebar approach then you will have to do a lot more measuring when you build the wall.
Then of course there is the option of trying to find some cheap dirt. If your lucky, you might find someone who has recently excavated a property who doesn't want the dirt. They might even pay YOU to take the dirt off their hands.