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