state a plan of action without seeing this project. I rarely say that with a post, but here you have either a heavy sanding project, or a paint scraper draw knife tool job, or re float job.
I am sure you have tried the first task and found that it really gums up the paper real fast and does not work well. This is the deal.
Now, I would recommend that you try a special painters paint stripper tool. Its totally manual and works on the principal of applying the knife edge blade on the surface and drawing down on it. This nicely makes a scraped swath. You can get this tool in up to 3 inch blades.
Now, you could do the easy method and have a texturer come in a re float the entire room. That means applying mud or joint compound on the walls and troweling or floating them out. The result will be new perfectly smooth walls. This method is expensive, as you can guess.