Just for !!fun!!, I tried invading hell with nothing more than pure military. I had 15 super axe lords and sword masters all in full adamantine. Most of it masterwork too. Normally, these guys could chop a full size fortress defense mod siege into pieces. Every single combat skill was at legendary (even biting and misc. object user, teacher and student were up around master too). They didn't have too much trouble with the demons at first (no syndrome breathers anywhere, incredibly), but everyone started melting and then bled out. Copper ore on the ground melted too.
I save-scummed, and tried again with a little more strategy. I only sent one squad in at a time. I managed to kill 26 demons out of about 50 before everyone melted.
I save-scummed, carved a passage to the surface, and then unleashed the demons. I was planing to let them walk through my entry defenses (lots of steel spike traps!). They got to my entrance (took forever, framerate=0), stood on a steel drawbridge, and started beating down my steel front doors. After finally breaking down the doors, a fire demon (forget the exact name) stepped (flew, whatever) onto a spike trap and exploded into flames. Molten steel everywhere!
Trying yet again, I turned off temperature and sent my military at them again. This time, we were victorious. All but one of the demons died (one had its wings chopped off and was stuck in the adamantine tube), though most of my military was dead. One axe lord, very conveniently, had somehow fallen into hell without any injuries so I had him explore. Only two demons down there, they ignored him till he got close.