Resistance > Avoidance. Resistance smooths out damage spikes. If you have atronach stone, a 1-shot kill will still 1-shot kill you, it just may take one or two more casts.

Alteration Tree has magic resistance and magic absorbtion in it. Pair that with Beton and Mara's quest and Antroch stone, most magic doesn't bother you much anymore. Not to mention the armor spells help you survive.

Look for items with + Resistance enchanted on them. They will save your life. Far as 2H bosses 1-2 shotting you, all I can tell you to do is turn down the difficulty.

Here is a tactic I use while playing as my one hand/shield warrior against mages. It works particularly well in dungeons but can used in open areas and works best with cover you can get to quickly. First I find cover or a blind corner. Then I set a lightning rune, equip my bow and begin stalking in sneak. Once the target is in site I use my resist poitions for me then place the best magic damage posion I have on my bow. After the first shot I move back to cover or corner and equip my bound sword and shield. Once the rune goes of I jump around and use either unrelenting force or ice form then place another magic poison potion on my sword. While the mage is climbing to their feet I blitz them making good use of my bash perks.

You know for a game with boring combat and no strategy this thread still got 26 posts with differet methods and opinions on how to fight...

Depending on your alchemy skill and perks, you might be able to craft elemental resistance potions powerful enough to get you to the resistance cap (up around 80% or so I think.) Carry a bunch with you, and make it a habit to chug the appropriate one(s) as soon as you see a spell of that type being cast. And make sure you harvest any snowberries you see, they can be used as an ingredient for any of the three resistance type.

Spell damage ignore armor. And blocking with a shield only helps with the specific perk in this tree.