Steel carapace too strong!

[dummy]

Is it intentional to have the game randomly generate monsters with modifiers so strong that no manner of weapon, spell, et al can damage them?

We routinely encounter several monsters throughout the game that are impervious to any manner of damage.
Not only is this a thoroughly irritating practice, but it leads to a question: If one of the main functions of the monsters is to allow the possibility of gaining experience points, aren't the "impossible to kill" not only a practice in frustration but also patently preposterous since it is completely antithetical to the very purpose of the monsters?
Stong monsters are an enjoyable challenge, invincible monsters just plain, well, suck.

Could this be modified? What are others perpspective on this?