What should I do to mitigate heavy spell damage?

I keep running into encounters where the spell casters kill me. In particular frost spells seem to be the worst. It's not fun getting 1-shot from 300 hp by a spell. I haven't done anything to protect myself against spells, compared to the herculean efforts I put into armor. What should I do?

There are several methods to increase your resistance against those attacks. You can increase your magic resistance, which works against all kinds of spells, and you can increase specific elemental resistances.

The following ways can be used to increase those resistances:

• Enchanting your armor with magic or elemental resistances
• Using potions of magic or elemental resistances
• Up to 30% magic resistance from perks in the Alteration skill tree
• The Lords Stone, grants 25% magic resistance
• 15% magic resistance from finishing the quest The book of love
• Some starting races have elemental or magic resistances (e.g. Nords have 50% frost resistance)

Having conjured minions or other followers can also help to divert the attention of the spellcasters and give you the time you need to neutralize them.

"When confronted with a wizard, ...close the distance and hit 'im hard."
- Lady Allena Benoch, Words and Philosophy

If you're investing perk points into Armor and/or Blocking skill perks, these should be useful when going against magic-based attacks:

Elemental Protection, a Block skill perk has a 'Reduce incoming fire, frost, and shock damage by 50% when blocking with a shield' perk. It requires a skill of 50 in Block and the Deflect Arrows perk.

The shield, Spellbreaker, has a 38 armor rating and has a 'Ward - Deflection 50 pts' enchantment. You can get it from 'The Only Cure' quest.

The Dawnguard DLC adds the Aetherial Crown to the game. It is one of the three possible rewards for the 'Lost to the Ages' quest:

It retains the power of your old Standing Stone power when you choose one, meaning that while you wear it you can have two Standing Stone powers active at the same time.

With it, you can activate both The Lord Stone (adds 25% magic resistance) and The Atronach Stone (adds 50% spell absorption).