I was really hoping for some engaging content, but it seems like the devs said to themselves "Hey, dark souls is popular.  Does anyone know why...?  Oh!  It must be because players love dying in 1 hit!  Let's roll with that!"    I know I shouldn't have expected much after the "puzzles" in the Magnus quest line, but damn, I'm tired of being disappointed by everything new that I try in this game.

Am I doing something wrong here?  Am I not supposed to be getting 1-shot by calamity lv 1 bosses?  is 390 DP on a guardian not enough?  Do I have to reroll to an OP class?

Have you been managing your aura and light orbs?

Also, as in Dark Souls, you do actually have to pay attention to the mechanics, as a number of the oneshot mechanics apply no matter the calamity level. 

But yea, as Entropoid said, your DP is pretty much worthless, as only 10% is applied. The light orbs are much more important to use.

Your gear is only 10%. Rest of power comes from aura fire, moon or earth.

There's a good guide on blackdesertfoundry explaining the mechanics. The monsters have voice cues or animations that you have to pay attention to. Learn the mechanics and those 1-shot hits they dish out can be avoided easily.

Black Shrine Blitz Boss Guide

Ok that is very helpful.  I completely missed the massive tutorial in the game that explained all of this.