If he has 50% resists in hell, and your mastery is -200%, he will have -100%
If he has 75% resists in hell, and your mastery is -200%, he will have -100%
If he has 95% resists in hell, and your mastery is -200%, he will have -100%
If he has 75% resists in hell with other stacked resists(4 thul shield), he will have like 200% to all resists in theory. So what you have to do, is shave off his current resist(75%) and keep shaving it until all 200% is gone.
So, if he has 200% resists in total, and you do -200%, he will have 0 resists to cold.


Cold mastery does not effect cold immune monsters.