Most people use a chain whip to get cogs off, you can probably get one for about 15 bucks from your local bike shop. If you are applying pressure to the lockring remover in a weird way you might chip off bits, otherwise, I'm not sure why it would do that. give it a shot with a screwdriver and a smacking tool (ie. hammer, another screwdriver, rock etc.) I believe you could also get the cog off by holding the wheel and backpeddaling, though I'm not entirely sure how healthy this method would be. You could probably also just use a piece of chain and a leather glove for hand protection and pull on the cog with that for grip.