To the OP: You can do Labyrinthian at any level as a mage, as long as you have at least a fire atronach... I did this on master at level 11-12ish, I believe. Also, having the Atronach sign active for spell absorb is an incredibly good idea for the very end, when you face Morokei.

99% of the enemies in that dungeon are undead, so the fire nach rips them a new one. You don't even need fancy high-level spells... just Flames will do just fine, acting as AoE support fire for your nach. The most frustrating thing for me in that dungeon was the priest's "magic blue voice" that kept svcking out all my magicka every time he taunted me as I progressed further down. I definitely did not rush through the levels, as one utterance from Morokei would leave me defenseless. Having zero magicka and a bunch of draugr surrounding me would have been quite unfortunate.

-Loth

Edit: I should also add that I did not have any enchanted gear for that playthrough, except for some novice robes to counteract the stone's magicka regen penalty, and the mage circlet you are given before the dungeon at the college. I think I might have also been wearing the magicka fragment of that necklace you get at the end of the Saarthal mission. No, now that i think about it, I was definitely wearing that... so another +30 to magicka total, plus the bonus from the circlet, which was +30 as well (I think).