Being level is not a strict requirement, being flat is. It is definitely convenient to be level (your pencils don’t roll off, and you can use a level to balance out large uneven slabs you are flatening), but not required.
You can always put the low side up on blocks (like some plywood or particle board laminations). Make sure they rest on the floor solid. If they rock or wiggle, put some caulk or something between the blocks and your floor, and maybe between the feet and blocks too so they don’t slide easily.
It would depend on the amount of “out of level” it is. A short distance then I would use longer feet rods as mentioned above. However, very long rods may lead to bending of the rod or machine stability issues. So perhaps the better route would be packers made from a material which does not compress to much. ie hardwood or plastic.