I feel like a bit of a barbarian, when I superglue an M10 nut (other sizes will also work) to the back of some particularly gunked up, cross threaded, corroded or mauled case back, but it does work, and if you take care when removing the nut/glue with acetone, the case will be none the worse for the experience.
I do have a couple of case back removal tools, but sometimes you are up against damaged notches or so much corrosion that you fear the tool may slip, or something will break. Don't try the superglue and nut trick on soft precious metals or some forms of plating, as it may not end well. Stick to stainless backs. One other obvious advantage is that you get a helluva lot of M10 nuts for £2,000