I breached hell for my first time today, while trying to mine out adamantine for my military. I didn't realize there would be a 5 z-level open space the demons could fly through, so I wasn't prepared at all. I drafted around 80% of my population and sent them to guard the passage to my fort. The demons started poring through, a wide variety of otherworldly insects.... I thought I was doomed. Surprisingly, my untrained recruits actually killed quite a few demons; most of the slush mosquitos, blue sky monsters and mayfly brutes lay dead. The salt and soot monsters, being unkillable due to the material overhaul, eventually overwhelmed my dwarves, but all the organics were slain.
Later, I sent an adventurer to visit the fort. His first kill was a giant olm wandering about. Second and third were another slush mosquito and mayfly brute. The salt monsters were still marauding about, but I could simply chuck a couple rocks at them to critically wound them and they would lie prone, and I could just walk away. I continued exploring my fort, and after killing around 15 demons and wounding many of the unkillable ones, I only had two yellow wounds. I only died because my adventurer decided to jump away from a salt monster's attack, right off a ledge into a 12 Z-level drop.
So... why can unskilled people wound/defeat these things so easily? I almost think they wouldn't even be a threat at all if not for the unkillable inorganics, which is more of a bug than HFS' ferocity.