OK, the obvious example being used is Hydross.

Can someone direct me to a write-up, or give an explicit description, of how resistances are calculated for a mob that does elemental damage? Alternatively, give the hybrid example of how a mob that does both physical *and* elemental damage works?

Basic assumptions:
1) Round resist to the nearest 5. So either 300, 305, ..., 360, 365, 370, etc
2) Amount of defense is known, and can either be sub 490 or precisely 490. Anything over 490 will be ignored.
3) We're dealing with a warrior. Let's just ignore paladins and druids for now.

Let's assume full protection spec, with 3 points in Improved Def Stance (for 6%).

How does an elemental hit work, and where are the resist checks done? What is the chance of getting a 0% resist? a 20% resist? a 25%? etc, etc.

If there's a write-up somewhere, point me in that direction and /lock this thread, please. Thus far, I haven't been able to find one.

Resisting Magical Damage and its Relation to Resistance Levels has some excellent discussion on the matter.

Excellent, thank you.