Cold Mastery

To add to the above description, Cold Resistance cannot be lower than -100% at any given time. So an enemy with 95% cold resistance near immunity being struck by a skill with level 36

Cold Mastery which reduces resistances by -195% would have it's resistances set to -100%. Making the absolute maximum level of

Cold Mastery ever needed to be level 36. Anything beyond level 36 would have zero effect regardless of initial cold resistance since resistance cannot go any lower than -100%.

Hell

Mephisto has a cold resistance of 75% making the maximum level of

Cold Mastery needed to effectively farm Hell

Mephisto level 32. So any skill points used to exceed level 32 are going to waste if your only intention is farming

Mephisto. Your maximum effective level will vary based on the chosen area to farm. You may also consider maxing other skills before

Cold Mastery if your build can benefit from skill points elsewhere first. A good base target skill level for

Cold Mastery on a fully geared Sorceress is commonly level 17, the 3 additional skill points are better spent elsewhere in most cases.

Cold Mastery Damage Multiplier Table (i.e. 75% opponent resists = 1/4th damage)

Cold Mastery only works at 1/5 effectiveness against sundered monsters. Therefore it makes sense to maximize

Cold Mastery to increase your damage against these monsters when using

Cold Rupture.