1. First, put some fine penetrating oil around the join between the case back and the case and leave for a while. Then attempt to open with a ball etc. Cigarette-lighter fuel has good penetration as an oil and worked for me once.
2. Place a cube of deep frozen ice on the case back to minutely shrink it and try opening with the ball etc.
3. Super-glue a large nut from a standard size bolt to the case back. When the glue is set hardened (24 hours), carefully undo the nut with a socket spanner. When opened, soak the combined nut and case back in acetone, which will soften and remove the super-glue. This has also worked for me. Be very careful about inhaling the fumes - best done outdoors.
My 3 pence worth.
Clive