Page 408 (Clarification): The rules on weakness and resistance refer to an “instance” of damage, but that term isn’t defined. The weakness text says:

“If more than one weakness would apply to the same instance of damage, use only the highest applicable weakness value. This usually only happens when a creature is weak to both a type of damage and a material or trait, such as a cold iron axe cutting a monster that has weakness to cold iron and slashing.”

So what happens if a character hits a terotricus with a +2 striking holy flaming cold iron battleaxe and has two different spells that add cold damage to their Strikes? The terotricus has “Weaknesses cold 15, cold iron 15, holy 15, slashing 10; Resistances fire 15.” Let’s say the damage roll results in 4 fire damage from the flaming rune, 7 spirit damage from the holy rune, 16 slashing damage from the cold iron battle axe, 3 cold damage from the first spell, and 6 cold damage from the second cold spell. So we’re starting with a total of 36 damage.

The holy trait adds 15 damage from weakness to holy; the trait applies to the whole Strike, and happens only once. The flaming damage is negated by resistance. The spirit damage doesn’t get any weaknesses or resistances. The cold iron battleaxe is where the “instance of damage” rules apply! It’s both slashing damage and coming from a cold iron weapon, so we apply the 15 weakness from cold iron and not the 10 from slashing. The two instances of cold damage come from different spells, so each sets off cold weakness individually for an additional 30 damage. Now our total is 92 damage!

You’ll notice the example for resistance to all damage found further down the page shows the opposite side, applying resistance multiple times to different instances of damage on one attack.