This sounds like the Wizard | Body Magic | Offensive | Disease spell, which reduces all primary stats. AFAIK the spell can not be resisted, though you may be able to use potions to counter the effects. BTW, it can be learned in act 2 from a ghost in the imp village, in act 3 from the temple mage merchant or from 'Zandalor' on the ruins island, and in act 4 from a book in the summoning level.
There are individual and party based Cure Disease spells, which have lower mana requirements than the offensive version (learn from Annabelle in the BF of acts 3-5, or Elder Ansukar on the summoning level in act 4). Warriors probably do not have a lot of mana, but they may still be able to counter the spells of one or two mages. Even if you can just buy yourself a few more seconds or so to attack and defend at full strength, it might make a significant difference.
Are you playing two melee fighters? If so, they would be at a bit of a disadvantage against ranged opponents, particularly mages. If you can switch one to a bow or crossbow (even just as a backup), that would let you start hitting opponents sooner, which will interfere with their attacks. The act 1 summoning doll can equip a crossbow. Even leveled up he is not very tough (note: summoning doll upgrade skills can not be unlearned), but as long as he is out of the way so opponents do not target him he'll last through the fight. If he does get targeted, he will at least provide a temporary diversion for your main characters.
Other than that, if you approach slowly you should hopefully be able to take on a couple mages at a time. If you run into a group, you could withdraw, and those that followed you should spread out a bit, and you might be able to turn and fight the closest, while the others were still catching up.
