got one shot (beginner)

Hi, I was running 2.7k like and 1k energy shield dual wielding on act 3 merciless. My resists are capped except for -60 chaos res. Got one shot (in HC) by the "rain" attack from perandus chest mobs; trying to figure out what went wrong. Is it because of chaos res? Not enough HP?

That is not the highest amount of HP, but main problem is probably lack of physical mitigation.
Elemental resistances will not help you against physical-heavy attacks, like Rain of Arrows.

-60 chaos res on hardcore is pretty suicidal, too. But this might not have been the case, this time. :v

Thanks for the reply, I usually use flasks to mitigate phys, but wasn't active at a time - didn't react fast enough :/

You might want to try using Arctic Armour or Chaos Golem (Endurance charges, if you have access to those).

Late game, you pretty much need to have some sort of physical mitigation. :V

That seems like really low life+ES in Merciless. The hardcore players I know that get high level characters would be playing with 4500 hitpoints minimum for where you're at. Hybrids are nice but I'd focus on getting at least 3500 pure life before looking to raise the ES. Status ailments and stun are all calculated off your life.