At basic level you can use the high res you made and just make a low res by hand just like done with the orc and bake the details without the multires taking part. Probably just apply it so it makes a high res to bake down from.
Multi res used to have problems baking down like it was meant to, I think Grant mentioned it in the course, but it was supposed to be generally fixed.
Bu the basic way should still work and get you forward, using the sculpting you have done already.