Generally, a mage should do significantly higher damage than a warrior because:
1. They have significantly less armor and health. Bosses and stronger enemies can two shot mages.
2. They are more limited by their resource(magicka) then a warrior character.
For people saying "buy better spells", imagine that you're regular swings no longer inflict significant damage after level 40 or so. Imagine that you have to use power attacks ALL THE TIME to do significant damage. Now you should see the problem. Higher level destruction spells have magicka costs through the roof, even perked up, and will never do as much damage as someone with a one handed sword and +100% damage from perks and smithed/enchanted weapons.
Are weak enemies one shotted by my fireballs? Yeah. Do bosses and other enemies take an absolutely [censored] number of firebolts before they're down? Yeah. While fighting a dragon priest, I had to run around four seperate times waiting for my magicka regenerate so I could even do ANY damage to him. It took FOUR mana bars to down him, and I have almost all my level ups into magicka, am a high elf, and had good gear. Seems a little silly to me.
The other schools make up for it some, but most bosses will kill your summon in a few hits, and conjuration magic is incredibly magicka intensive. Alteration is pretty moot for battles.
So having two conjured flame atronachs that can dish out damage probably comparable to a high level player character, and you can get TWO at once, then on top of that you as the mage can dish out pretty decent damage to augment them with destro powers..
All in all the damage you as a mage can put into the "battlefield" just playing the mage is the accumulated damage of two atronachs and your own damage dealing powers.
I'd say that is probably more than TWICE the damage a high level warrior can dish out all alone.
As for the atronachs dying by a boss, first off get the perk that makes them have more health, secondly it is a BOSS, it's not suppose to be a cake walk.
Especially if playing on Master.
You complain about "FOUR MANA bars" to take down top level boss.
How about you ask a Melee warrior how many times his stamina bar is depleted with just one or two power swings? It also needs to regenerate or you are just doing base damage.
A warrior would also have to run and hide and use restoration spell or potions to stay alive, they also need to get CLOSE to actually do damage.
A mage can KITE around and do it from a far.
It seems silly too ME that you think one of the toughest bosses are hard, is that how it's suppose to be?
If you are playing on high difficulty level EVEN MORE SO.
Seriously people think before you post...
I get a destro mage alone is not powerful enough, but this is why you have other magic to use.