Doesn’t matter how tough they are - once they run out of ammo, they have short, not very ergonomic, spears and armor that isn’t very effective against blades, bludgeons or arrows.
Unless they made friends (marines are pretty good at that...), it will be over very quickly and very messily - esp. if they ran into Mongols.
Lol. I know how to make explosives from piss, feces, and ####. And I’m not a Marine.
They are trained to adapt. They’re very good at it. And they have an extra millennia of knowledge.
Seriously, it would not even be a contest.
And with respect to the current discussion, you don’t think alien invaders would understand the concept of supply lines?