I have no idea what is killing me/dealing massive damage

I died in Abyssal Depth via a Rare Mob.
I was fighting the rare mob, and I was intentionally going in and out of fight to get a quick hover over one of the debuffs with the mouse to figure out what they are doing but they often only appeared for fraction of a second and I'm constatnly trying to avoid damage.
When I tried to stand still and actually read the appearing and disappearing debuff icons, I died.
It's impossible to adapt to these mobs if I don't know what they are actually doing and their huge DOT or Insta kill mechanics stay a mystery and it's very frustrating because I don't how to deal with it.