Well if it was me, I would reconsider the design of the wall, if it is no longer having running water over it.
I would go with a more natural look and use as big of boulders you can handle...basketball size? Slope the bank enough so they do not topple in with time. Mix these with landscaping plants to help hold the dirt in. Still put a drain across the top ridge to prevent as much erosion down the slope as possible.
Carrying enough concrete in to make a footer just doesn't appeal to me. My back just couldn't do it. However as stated above, it is the best way (and just about the only way) to go for a true wall to stand the test of time. Side thought - instead of carrying all that concrete, I would be tempted to try engineering a big muck floating pad like an excavator would use to go across soft ground. It would have to be something like twice as wide as the height you need, then the wall fastened to it....
