Many people (including myself) consider this method to be cheesy, but i'm happy to use it in moderation if it means achieving the kind of power that a high level mage SHOULD have.

Get yourself a decent alchemy skill, stock up on Bloat and Ash Yam and make yourself a batch of 5 fortify intelligence potions. Drink 'em and then make 5 more and so on. The magicka gains from The Atronach sign pale by comparison.

Be it stealth, combat or magicka, primary attributes are where the power lies in MW. And alchemy is the unbalanced skill that can harness them all...

isnt there a spell that makes you drain magica and give it to yourself? In that case, you could make an attack-spell that at the same time gives you more mana than you used for the spell?