There is a quest where you can get a daedric shield, which blocks most spells. Also I use atronarchs ( fire, frost, storm ect) to soften enemy's in huge numbers. When in battle with tuff mages, I use the spells like shock, frost, fire,. But the spells ( sorry cant recall there names) that you can cast on the ground, and whatever enemy walks on them or hangs around gets cooked, shocked, or frosted. So with flames burning on the ground continuously, you can vanquish any strong undead enemy. I use the same spells for mages like shock. I place the shock spell everywhere then the Mage will either walk around over the areas you have placed the spells, or hang back, where I just sap there Heath with long range attack spells, or send in a atronarch. Also I'm a powerful archer. So if all else fails I shoot, and run, shoot and run. I can bring down a powerful dragon priest with my bow alone ( I did last night against a very powerful dragon priest on the dragon born DLC). So I think it doesn't hurt to branch out in other skills besides magic. I did and my Orc is one tuff Mage/warrior/archer hehehe.
Also save, save, save. Before any battle with powerful mages. Sometimes I have to die a few times, trying different tactics, before my opponent is finally crushed.