Its not like shrinking a dress is magically going to make a smaller size.
You have to take into consideration the grain, the seams which might not shrink with the dress, and any fancy type of treatment to fabric, interfacings, zippers, etc.
Your seam lines could droop, the interfacing could pucker, finishes might not lay flat. It could be a world of hurt and you could ruin the dress.
If you were only a size below the dress, I could maybe see it, but we are talking 4-5 sizes. What you are thinking of doing is just unrealistic.
I would personally say sell it to someone close to that size than risk ruining a garment that can never be replaced.