I like to use my bow, but I won't use it when I have an undead army on my face.
If there is undead army on your heels then one mage makes no diference

If you really mean "army". And you did something wrong to get in to such situation in the first place.
Good point, but I haven't found any object with resist magic effect. Also it's really hard to use a shield with battle axe - I have to first find a spell which gives you third hand as constant effect.
There are several ways to combat magic in the game:
-sneak: if you can kill mage before he sees you. Sneaking combined with archery makes wonders. You can shot, hide and repeat lot of powerful enemies. Just keep your companion somewhere where he can not give you away or charge bravely while you try to hide in the shadow.
-potions/alchemy: potions can give you resistances to magic.
-poisons/alchemy: there are poisons which damage magicaka.
-magic: there are spells which protect against magic or damage magicka on target.
-enchanted weapons and armour: there are enchantments which protect against magic or which damage magicka on target.
You can of course combine them out: dip your arrows in magicka damaging poison while sneaking on your mage having helmet of resist fire on your head.
It comes down to your character build. In this kind of game, very specialised characters does not last long.