Honestly I'd do a resistance method of "fuzzy" amounts. Then your rating determines the power of the attack needed to bypass, partially, the armor/enchantment and damage it. For example:

Armor of strong fire resistance.

Lets classify that as about 1500K in temperature. Obviously it is enchanted/made to resist those temperatures so exposure at or below that level should cause no damage except in prolonged exposure (like burning other armor due to the duration). However, exceed the level and the armor would quickly come apart. Like a variable resistor given too much current ... fzzzzzzz, POOF!