Bracing a wall?

Hi, I have a houes built into a sidehill. Dug out area is 6 feet at theback wall and tapers down to 2 feet in the front. I was told I would have to have the back wall braced or supported someway before I can backfill it. Its not a concrete basement but treated plywood. Area is all sand and well drained. How does one make a support for that back wall.

Thanks.