Temporary brace for a wooden retaining wall
No need to purchase special braces, just use some common lumber.
When i set fence posts I use a 2&quot;x 4&quot; and wood stake.
Lay your 2x4 on your wall at angle, the farther the end that will be at  the ground/stake is away from the wall the more lateral stability you will have. Secure the brace to the wall, you may need to add some scrap wood to attach it to, another 4 foot 2x4 screwed to the wall vertically will give you a good attachment point.
With brace in place set your stake at the end of the brace right up against the side   and use a heavy hammer to drive in the ground solid. Now you can screw the brace to the stake. It is best to pilot drill the stake so the screw does not split it.
Tutorial for setting fence posts
Photo from link.

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