I'd run your mage as your main, just personal preference really no explanation for it.

For your Priest you can use something like:

/cast [target=focustarget, harm] Smite, [target=focustarget, noharm] Heal

Setup your Mage as your focus for your Priest. If the mages target is harmable it'll smite, if its not harmable it'll heal it.

Also you could just healbot with your priest and AoE Grind with your Mage as Ice Spec.