I don't do dual cast/impact. Mostly for Magicka Efficiency, you get more bang for your buck doublecasting, and just spamming the same spell over, and over gets old pretty quick. I also don't Enchant up to free magic, because Bethesda put Magicka in the game for balance, so removing it makes it all too easy. I know, the standard advice all over the 'net is "Fortify Destruction 100% dual cast Impact" but that's not really playing a pure mage so much as a glitch suit turret. It's a no brainer, and Pure Mag is supposed to be the Intelligence build.
Instead, I like to combo different spells, my favorite so far is Ice Storm, and Flaming Familiar. The latter is pretty much a fireball on legs with a 6 second fuze, and the former detonates him on contact. Now, there isn't much of anything significantly resistant to Frost, AND Flames, you get the DOT from those still burning, and slow from the IS.
Also, both can be used indirectly. FF will run around corners, while IS goes right through cover, so that annoying archer that keeps ducking behind a rock? Tell him to Chill. It really helps to use Aura Whisper so you can see what you're aiming at, though. Cover is even better than capped out armor, because it stops all damage, and they can't see you, in case you're feeling squish in just robes.
I also like to kite enemies onto Runes, and/or Wall spells. Just because it's called "Wall of Flames" doesn't mean you have to use it likeat. An Altmer in robes with nothing but magic in her hands runs faster than any other playable character, so you don't even have to sprint, nor look at the enemy. Just cast them at your feet, and run over them, with enough magicka, Giants are relatively easy if you're patient.
Destruction caps low, but you can prevent your character advancing by limiting the number of skills you level/perk. You don't Have to go up to 81, nor even use every school. Pure Destruction is a little light on the defense, but paired with Restoration, you can Ward, or heal. Alteration is okay on defense, but I always find myself dying when it wears off, and I lack either the time, or magicka to recast before being 1 shotted.
Illusion is just fun. You can't use it alone, but with a decent follower, and careful strategy, you don't have to fight anything personally, ever. Invisibility, and Muffle can completely replace stealth, and both still work on everyone. You can alternate casting Calm, and Firebolt to wear someone out (not unlike Impact turret, or Hack&Bash.) Conjuration can replace, or add to your follower to kill the survivors of the Frenzy moshpit.
My most powerful Pure wizard is BAMFO, Illusion, Destruction, and Conjuration. Illusion is mostly for Invisibility, and Frenzy. Destruction is almost only for Augment Flames(2) to enhance Fear, Flaming Familiar, and some enchantments. Conjuration lets you "Teleport" your Dremora Lord by recasting him where he's most needed. Meanwhile, you drop Invisibility, and have the running speed to relocate before it wears off. Wuld is also close enough to instant, so he's pretty much a teleporter. (Hence the name.)
Not unlike a Nightblade, only instead of using Illusion to backstab individuals, he takes on entire armies without being targeted, much less hit. As soon as you break cover to recast, Wuld, and drop Invisible again. Maybe throw an Ice Storm to slow everyone down, or Frenzy to clump them all together, but that's basically it. Easilly built below level 30. No armor, 100 health/stamina, and an almost inexhaustible supply of Magicka, he's just as deadly in a loincloth.