I've googled, but I can't seem to find a definitive source for this. Much of the damage in the Plagueworks wing of ICC is classed as Shadowstorm, a combination of shadow and nature damage. How does resistance gear work against this class of damage. Is it:
a) The sum of Shadow resist + Nature resist is used
b) Shadow or Nature resist is used to calculate resistance, whichever value is higher
c) The average of Shadow resist and Nature resist is used
d) Shadow or Nature resist is used to calculate resistance, whichever value is lower
I've certainly seen it speculated that it's A, but I can't find any specific statement or testing to confirm.
Mimiron's spellfire damage and Anub's freezing slash are both examples of mixed school abilities that don't benefit from resistances based on one "half" of the spell. I've seen spellfire damage partially resisted, but this is due to base resists or "all school" resistances like MoTW, not fire resist. I may be 100% wrong, but it is a possibilty.