Best way to level a room and lay temporary flooring
Thin plywood will only flex and make noise. Anything on top of shims must be able to carry the load of furniture and foot traffic just as a standard subfloor must. 

If your floor slopes gently just half an inch, there's probably no reason to do all that work. Most flooring will accommodate that. Lay your temporary flooring on what you have. If you do need a better substrate for the particular type of flooring, lay some inexpensive 1/4" OSB or mahogany plywood to provide a smoother surface.