22kw generator mounting pad on slope
You will need to dig a footing below the frost level , you don’t want the pad moving around. For diy a easy way to build a concrete wall is with Cinder blocks the hollow concrete blocks you stack like...  blocks.
Some rebar dropped in them and back fill with concrete can support just about anything including my truck when we brought the generator in.
I have even used bagged sackcrete on just such a small retaining wall and I mixed it in a wheelbarrow a fair amount of work but inexpensive and solid.
After the footing and wall is cured backfill with dirt or rock. I poured a small pad on top and that powers a remote cabin generator so we had the comforts of home when we went out there.