Iosue
Legend
All these monsters are Challenge 2 (450 xp) and all average 12-13 damage for their basic melee attack, 10-11 for ranged.
[snip]
Given how much higher the monster hit points are I think it's pretty fair to say that a Level 2 party isn't likely to be facing more than one of any of these at a time. 4 Ogres would easily live long enough to murder the entire party.
With that in mind I'm almost certain that Challenge # is basically the absolute lower limit of where the monster goes from being "almost certainly lethal" to "only mostly lethal."
We know that Challenge Rating in 5e represents a "moderate-to-challenging fight" for a party of 4 of that level. So this seems to fit pretty well. 1-on-1, yeah, a level 2 fighter is going to have a tough time. With a fighter and his friends, though, the ogre is way outnumbered, and is probably going down in relatively short order.