This concerns me.
I was planning to be a magic damage dealer. But seeing that the enemy could have an elemental resist AND having armor vs magic, I feel mage will become useless in late.
That may not be the case in the first act where enemy don't have much resist. But later on I fear that they will ALL have some magic resistance which will make your mage deal WAY less dmg than any physical.
At this point i think the only viable party is to make full physical damage. You won't have to bother about magic armor or elemental resist. And you will destroy armor fast enough to use cc on the enemy.