what weapon would the be using?
Medieval or moders?
Even with medieval, I see them having a good chance to clear most of LMoP.
Just take is slow and get cover always.
but that is the problem, how do you define "real human" here?
real humans went through decades of war and some of them survived dozens of battles?
do they fight "real goblins" or whatever that would be?
or are real humans bunch of 20YO with no fighting skills whatsoever?
Skill and such only matters to a small degree. The issue is, and always will be, hitpoints. You mention that "as long as they get cover" they will be fine in Lost Mines, but if memory serves me there is a Nothic in that adventure. Now, maybe it is friendly or non-hostile, but here is the issue that brings up.
A Nothic can trivially deal 10 to 12 damage by
looking at you. Canonically, it rots the flesh from your bones with its gaze. Now, this can be completely nullified with a Con save, but in no war in human history have you ever had to face a monster who can turn you into rotted meat and bones with a mere look. And the Nothic is also tough enough that you can't kill it in a single blow. You'd need to get three critical hits in a row with a Heavy Crossbow to drop it.
And yes, people will say "but I would use tactics and planning" but... how? No metagame knowledge means unless you know this monster is in the manor, and how it works, you can't pre-plan anything. No spells means you need to carry any materials, and even if you blind the monster with smoke or something similar, you are blinding yourself just as much.
So, even if you ambush it, unless you ambush with all ranged attacks, all of them are criticals, and you roll high enough to kill it... you are going to end up on a coin flip for someone dying.
And Nothics are not powerful DnD monsters. There are stronger enemies IN the Lost Mines adventure.