There is no problem getting Campagnolo cassettes off. They use the same splined tool as Shimano in the middle . . . .You might be right - I just pick one out of my toolbox, and I think I use the same one that does Shimano frewheels, but it is easily available and cheap - unlike most Campagnolo tools!
@derrick isn't just possibly "might" be right, he is right, and your advice errs. The spline pattern is different between Campagnolo and Shimano (both 12 spline) lockrings. I hope you have not damaged your various lockrings forcing the wrong tool in. Lockring tools to fit Campagnolo and Shimano freehubs cost about the same (~£7) (provided you don't get a branded Campagnolo one). Provided you have three tools: one for a freewheel, and two for Campagnolo and Shimano freehub/cassette lockrings you're sorted.
The Park Campagnolo specific one is this:
http://www.parktool.com/product/freewhe ... bt-5-fr-11
Quite different from the Shimano specific one:
http://www.parktool.com/product/cassette-lockring-tool-fr-5-2
The Campagnolo splines are only 0.3mm with an OD of circa 22.8mm. The Shimano splines are 0.8mm with an OD of circa 23.4mm (NB measurements on my Campagnolo tool and a generic Shimano one).
The Shimano one won't fit properly as the mark/space ratio of the splines is wrong. They have the correct number of splines, but they are the wrong shape. Campagnolo splines are 50/50 whilst the Shimano ones are more like 40/60 - ie peaks are narrower than the valleys. An OEM Campagnolo tool fits just the same as the Park one but has tapers so it fits snugly: make sure it's pushed home securely. The splines are shorter so you have to be carefully it stays in (by putting the QR back in, for example) before applying torque.