Root Abyss Boss Mechanics

1HKO generally refers to attacks that have a good chance to 1 hit people, since the majority of people tend to have less than 20k HP.

No it doesn't...ohko is ohko regardless of circumstance. Up until now there have only been a handful of true ohko attacks because DR can actually be tanked.

(Ie, Vellum's fireballs do about 22-25k damage (forget exact number) which is an OHKO on pretty much anything that's not a warrior)

I have more HP than that and i get killed every time. I'll do some RA runs later with HB and see what happens.