Example:
Level 40, armor 520, hit points 470, Adept difficulty. Weapon bow, Mixture of light and heavy armor (2 of each)
A Briarheart hits the character and there is a death animation. Reload save and move close to Briarheart and save to be able to repeat battle over and over again.
Without getting insta-killed it takes Briarheart about 8 hits to take out my character. I see two more insta-kills over ~10 tests. I change to full heavy armor set (armor now 620) and run several more tests. No insta-kills at all and it now takes about 10 hits to kill me (as expected as I am at armor cap now).
My question to anyone else who has seen an insta-kill on a character that you know can survive multiple hits from that enemy. What was your armor level? What weapon did you have equipped? Is there a way to prevent insta-death? This information is important if someone plays in a hardcoe mod (restart if character dies).