[@princess]

I hadn't thought of factions- raiders come to mind; scrappy outlaws, some of whom have managed to survive on the surface and declare themselves the king of one ruin or another on the surface, and aren't too friendly towards the "moles".

As for killing Them... They're abnormally strong, at any size, 20 meter class and above are effectively kaiju; find somewhere to hide, and hope they haven't caught your scent. the "smaller" ones can be killed with sufficient artillery, but that is not in ample supply. Even before the fall of man, [i]some[/i] battles were won, but it was a bit like the opening battle of The Last Jedi; sure, some forces managed to kill a colossus or two, but the losses required showed it was not a winning, long-term strategy. Those who do have old world super weapons settle for disabling the beast in such a way that prevents pursuit; blow off a foot/leg, or topple a building on it, and run like hell before he digs his way out. Or, critically wound one when there's a plurality, and the others will turn and eat that one instead (because cannibalistic predators).

The Giants don't regenerate, and their anatomy isn't all that different from ours, the issue is scale- A skull that's 10 feet tall is going to be very thick, so even with a tank, head-shots aren't a guaranteed kill. Even if you try to go for the eyes, tank gunman aren't snipers, and They tend to close the distance pretty quickly once you have Their attention.

any other questions?