Basically what he said, but I'll put it in English for you.

Each level in Cold Mastery drops an enemy's Cold Resistance by 5%. Now, that's only effective to a point (usually about -170%).

It's useless for Cold Immune monsters, as the monsters are immune to the -Resistance effect of Cold Mastery itself.

Anyway, the best it will do is drop a monster's Cold Resist to effective -100, thus doubling the damage you're dealing.

For PvP, you can counter this by stacking a shitload of Cold Resistance on top of your 75% resist (usually accomplished via a 4-Perfect Sapphire Shield). Since Cold Mastery drops your TOTAL Cold Resistance, and not just your EFFECTIVE Cold Resistance, it'll just take away from the assload you just slapped on with that shield.

Hope that helps you out.