To optimise the advantages of the race, and minimize the disadvantages, I highly recommend Shieldmage with Spell Absorbtion. Start out with Lesser Ward and Mirmulnir. You can take the cart to Winterhold, and take a class with Tolfdir to get it free, then Fast travel back. Irileth will wait...
Even Lesser Ward will Block Dragonbreath completely, but also drains a lot of Magicka. So, use Dragonskin to absorb the Magicka from it (With the shield down) and throw Lightning Bolts back at him. Another detour you can make is out to the Rift for the Atronach Stone. With Dragonskin, you can get 100% Spell Absorbtion for a mad minute daily. Grind up your Restoration (Healing, and saving your potions) to 60 for Ward Absorb.
This is basically a Magic Tank at high levels, all you really have to worry about is Physical Damage, which can be handled with Impact, and Shield bashes. Basically, keep everyone off balance so your magicka can recover, so this is the most important effect unless you Enchant for free Destuct/Restoration. (Nahkriin helps) Destruction is higher priority, because you have to spam it, and even if you do get both free, it uses up about 2/3 of your Enchantment slots (It's nice having a Shield here for 2 more with Extra Effect.) You can double enchant Destruction with Destruction, and Recovery on the body to use 5 slots for 20%.
In combat, you want to use Cold with Impact, ice Storm is best to Slow, Fatigue, Stun, and possibly Paralyze armies en mass in open battle. In a melee, you want a Shield, and Wall of Fire/Frost, depending on the enemies. Cloaks work well here too. Bash to keep them off balance in your Areas of Effect, and cast puddles of damage in between. Wall of Flames does 75 damage over 20 seconds, plus Burn damage. That's 1500 damage not counting the bashes to keep them from running away, and you can just pulse out a puddle for minimal Magicka. Impact is a lot more expensive, but can help setting up the Melee you want before steping in to Bash&Burn while your Magicka recovers.
I've done it with just Destruction, and Block, on Master, and it happened to be a Dunmer instead of a Breton. For a more comprehensive build:
http://skyrimcalculator.com/#279975 You'll want the Atronach Perk only if you get the Dragonborn expansion to wear with Miraak's Robes, and Boots. You might want to pick up Azidal's Gauntlets too to stack with Spellbreaker, and Ward Absorb. Then, you could use Summons from Conjuration with Spell Absorbtion because the Ward is at will, and you can cast the Summon/s before popping Dragonskin, and switching to the robes. Between 2 levels of Absorbtion, Magic, and Elemental Resistance, you're damn near invulnerable, and it gives you Magicka you can use for Destruction. Switch from the Atronach Stone to the Lord.
With 100% Spell Absorbtion, you have no chance to cast a summon, but get the Magicka back. Even when you cast a Scroll, or use a Staff (like Sanguine's Rose) you can basically just use enchanted items like that to refill your blue gauge. You lose one of the most powerful schools that way, though. Wards can't get you up to 100%, but whatever doesn't get soaked up can be cut in half with Dragonskin right after you summon, then Resistances kick in... So, the choice is between 100%, or being able to use Dremora lords, and Storm Atronachs... Necromancy, and Bound Weapons aren't affected by Spell Absorbtion, so you could raise dead with impunity.
For Gear, here's a short list of desirables: Necromancer Robes, Steel Shield, Heavy Armor (with Robes, don't perk it) Wolf Helmet/Boots/Gauntlets, Blades' Shield, (Not very Heavy) Targe of the Blooded, Spellbreaker, Ygrammor's Shield (For Deflect Arrows) Legionarre kite Shield (Also for Arrows) Spellbreaker, Master Robes of Destruction, Archmages' Robes, the Gauldur Amulet, Savos' Amulet, Nahkriin, Rahgot, shouldn't need Otar, but He has a nice Staff. In Dragonborn, Ahzidal/s Gauntlets, Miraak's Robes/Boots, anything of Destruction.
