There is a counter spell (wizard | body magic | individual or party | cure decease), but no preventative spells or resistances AFAIK.
You could use lower requirement equipment until you kill the mages, or use alchemy potions to boost your stats (if you can find holy water for sale, you can make permanent potions).
Are you playing a ranged or melee character? If melee, it might help to boost the act 1 summoning doll and give it a crossbow, to attack the mages sooner. If you didn't use the DK's skill points to learn summoning doll upgrades, this might not be feasible.
You could always leave the BF and enter another rift where there may not be any mages.
