One shot mechanics only start at Pit 61, and only for certain bosses.
There are no one-shot mechanics from Pit 1-60, and seriously every class and every build can do up to Pit 60 if they put in the effort and thought about their build, and MW their gear to 4 at least (which is an easy grind).
This is pre-midseason patch.
After midseason patch I would say every class and build can do up to around Pit 80 no questions asked, if you build correctly.
One shots happening are mostly due to people who did not bother getting Life for their builds, and are hanging around Pit 60 with 20k life.