Getting one shot becomes less of a problem the more experience and knowledge you have of the game. Once you learn what all the enemies do, including which attacks are dangerous and should be avoided, it is not actually that hard to avoid the hardest hitting attacks as long as you are paying attention. The hardest part is simply losing focus since the game is long but danger can strike at any moment. From the point of view of someone with 700 hours in the game, and who has gotten two outlanders to level 60+ in Hardcore Elite (including one level 100), the damage output from enemies seems more or less fair. Obviously there will be some parts that are less fair than others but in a game where it is human vs a lousy ai you have to grant the environment some allowable degree of unfairness or it will be impossible to die which makes the game not very interesting.
Here's a quick list of things you should try not get hit by as an outlander in Hardcore Elite:
All acts: melee. Outlanders should be ranged classes and getting hit by melee enemies that should be limited to very rare mistakes and the occasional charger.
Act 1 Part 1
- bear traps
- more than 1 melee guy
- Grell's cannonball
- Large waves of blue projectiles from possessed sturmbeorn
Really there's not much in the first part of act 1 that can one-shot you. It's the first part of the game and it is easy.
Act 1 Part 2
- Chillhoof's ice wave
- The ice elemental's ice wave
- More than 3-4 ice bats
- The winter widow's smash.
- Any smash / stomp attack, actually
- More than 2-3 chargers
- The mirka frostbreather's ice breath
- The mirka caster's star-shaped ground attack
- More than 2-3 poison bolts from the tentacle casters
- The exploding tumors tossed by the netherrim plague guys
I could go on but this list is just an illustration to prove a point, which is that once you know what it is that does enough damage to kill you, you have a better chance of avoiding most of it. This game is usually quite fair about making damage somewhat avoidable.
TL;DR: The game is totally playable in hardcore elite without getting one-shot. The strategy is to know what hits to avoid taking and have enough +HP on gear to survive a single mistake long enough to run away and drink a health potion.