It's not a DIY project. I've seen badly sloped sidewalks fixed by pumping sand/cement mix under the slab. They drill a couple of holes into the slab, pump mixture in at high pressure, and seal the holes. The repairs I saw moved large slabs 2 to 3 inches up to level. These were on sidewalks, don't know if this would work for a porch. I guess I'd look in the yellow pages to find a contractor that levels cement.