Umm, as far as i know toady fixed the unkillable demons, so they all die now, all the forgotten beasts too.
Used to hate getting one of the bugged blobs or shells that never died.
So you need like 150 steel clad dwarves in narrow corridors to fight off the initial hell invasion?
Would a lesser number work if you kill of the initial swarm with a repeating cave-inn machine?
Well, it's Dwarf Fortress. You can always find ways to exploit the game mechanics to do anything. I'm guessing you can use cave-ins and encasing stuff in obsidian to kill the demons without any bloodshed, but I don't find that particularly funny. No disrespecting the mad engineering feats I've seen, but I love the gorefest. And I can't speak in general terms for battling hell, it depends on the demons spawned, but of the four forts where I've tried to beat them, two crumbled and two won the initial battle, but this is the first one in good enough shape to rebuild and expand. Both of the ones who won against the demons had about 80 well-trained dwarves in steel plate, heavy emphasis on axes and swords since demons can't be stunned (nor crushed I think but I haven't verified), but it was still too small and I had to activate almost every civilian to finish them off, which of course caused 5 civilian deaths per demon, but in the end sheer numbers prevailed.
Edit: By the way, I had a legendary engraver that was activated while fleeing from a demon, he turned around and immediately ripped it apart with his bare hands. The battle was two lines long. It was one of the most awesome things I've seen. Then he ducked into magma.