There is a nice post on reddit about how damage is calculated in this game but I'll give the highlights:
#1) Calculate ability damage bonuses (ie: +20% damage to ability, +15% damage to ability) by adding all these modifiers together and calculating them to the ability damage [additive]
#2) Apply each damage reduction individually (ie: spell shield + anti-magic powder = 50% reduction then 50% reduction on the reduced damage ... so 25%). Or a cleaner way is basically take the damage from #1 and calculate the DR multiplicative.
#3) Apply damage modifiers after calculating the reduced damage (ie: Target takes 25% more damage) [additive]
It appears like debuffs like damage modifiers don't stack. It just takes the highest one so you can't just stack modifiers on a single target. Slows don't stack it just takes the highest one. Speed buffs appear not to stack, either.
Pretty clean and simple. Nothing to shocking but nice to know.