Yes, there are design issues here, particularly with how enemies cast all their spells at once.

As for dealing with them:
* Spell reflection has one advantage over spell resistance: If early spells get reflected, the enemy may die before it can cast the rest of its spells.
* Spell absorption can help if your remaining magicka is low enough. You might be able to buy an item that gives you passive spell absorption, so you don't need to worry about not having it up.
* If you have enough health to survive long enough, a powerful regeneration spell can help.
* On the other hand, if you do not have enough health to survive, you can use a Shield spell to block some of the damage. Shield spells work by giving you temporary hit points, and if the magnitude is high enough, it might be enough to withstand the onslaught of spells, or at least mitigate it to the point where you can survive the remaining damage. Don't forget that you can combing this with spell resistance or similar; the resistance may negate some of the attacks, while the shield allows you to survive the rest. (Note that it is cheap to increase the duration of Shield spells; at higher levels, you can get one that lasts longer than the time it takes to recover the magicka used, provided that it doesn't get dispelled due to damage.
* If you manage to survive the barrage, the enemy will have spent all its magicka and won't be throwing further spells at you.
* As always, save frequently so you can just reload if you get caught by a deadly onslaught of spells by surprise.

(Also, I believe the Breton's spell resistance doesn't apply to spells that do elemental damage, which I believe includes most attack spells; it's the same way in Arena and Morrowind.)