Nipple wrenches are one of the darker sides to c&b revolver shooting. There are two solutions: a wrench that fits the nipple perfectly or a hardened wrench.
The key, like with screwdrivers, is getting the wrench to fit the nipple perfectly. The wrench is always softer than the nipple, so any misalignment that causes local stresses will invariably damage the wrench. However, finding such a wrench is a crap shoot; you end up with half a dozen at $6 each, most of which you'll never use again. I have no advice on which wrench to buy for which gun; I've bought several and generally found ones that fit my various guns, but I can't remember which ones I bought from where.
Another option is to buy some Kasenite and harden the wrench yourself; this is not difficult but daunting if you've never used it as it involves heating the wrench to a high temperature.
Finally, modifying a socket as suggested above works if you have the tool to cut the slots. The problem with a modified socket is that it has fairly thick walls and won't fit on many guns. However, for those that it will fit, it's a good cheap solution.
