Sadly magic is weak on levels 30+, mages are [censored]. I figured you can lower cost of 2 schools of spells long time ago, but there is no enchant to increase the damage and mage damage is really really low. I play on expert and it's hard to kill anything compared to archer or a warrior. My guess is that at 40+ levels mage offense=0, your statement of mages having good offense is simply not true
(I play on the default diff, at higher diff its gonna be.... harder)
My pure mage, (no armor yet) is level 40. I just got the ability to buy incinerate, and thunderbolt. I still have no problem clearing out dungeons. Just because I cant one shot things does not mean its low damage, the AOE dps makes up for low burst damage.. WIth 0 cost or even 75% cost reduction in your destruction spells along with double cast and impact you can stun lock most everybody, you win because you dont run out of mana.
Here are my tactics for X number of melee badies:
I have a mage companion in full dwarven (non enchanted) armor. I gave her a bow that soul traps, a greatsword that soul traps and a dagger that soul traps. And alot of low value empty soul gems.
I try to keep ebonyskin up all the time for 200 armor to minimize the one shot kills on me.
I use my frost atronach (frosty) to tank most encounters and to bunch people up.
If my companion has not charged into melee, I can use a double casted fireball or ice storm to stun all the dudes attacking my frosty.
This usually kills my frosty, but I now am able to spam my AOEs to kill fighters who are all stunlocked.
Here are my tactics for 1 ranged mage/archer:
Use cover and dodge thier shots while I nail them (and stun lock them) with either chain lightning or ice spike or firebolt.
As always heal when needed, but healing potions are there to clean up if the stuff hits the fan.
I have just crafted a full set of orcish armor, and have enhanced it at a crafting table. I figure this will give me over 100 AR(no heavy perks yet). I will then enchant it to 0 the cost of destruction spells and crank up my elemental resists, reduce my alteration and restoration costs by 25%. This will move me away from a pure cloth mage, but thats the nice thing about this game. I have the option to go back if I want to.