For Sundering weapons or armor, I generally use the Durability factor from the Diablo game. I would figure out an durability number for a magic sword based on its enchantment level, basically a hit point range like a +5 Greatsword would have 250 hp and assign a durability AC 25 to it. So if something can sunder this Greatsword, it would have to make its typical Sunder attack versus AC 25 and if it hits the damage is taken away from the 250 durability HP's total. Once it reaches 0, the Greatsword would be unusable until it is brought to a blacksmith or some other place and repaired for lots of gold. If a creature uses a natural weapon to sunder, like a bite or claw, and they miss the Sunder attack very badly like on a natural 1 or by 5 or more on the AC, they instead hurt themselves and take damage for failing.