It wasnt just this time though, I have it happen a LOT where an enemy will just somehow kill me from full health in one hit.
You do realize that there is no enemy that can one-shot you from full health in one hit, right? As far as I am aware, the absolute most damage you can take in one hit is 49 (not counting getting knocked into lava). This requires:
1) Running into an Elite Former Commando (which only shows up in UV/N! Military Base or Ao100/ArAo666 in levels 80+)
2) Having said commando be equipped with a Napalm Launcher (3 different weapons)
3) Getting hit for max damage (It deals 7d7 damage, so max of 49).
And even then, you have 50 health at max, so even that cannot one-shot you.
Unless...flamewolf, are you not, by any chance, getting one-shotted on Angel of Humanity mode?