i breached hell and felt i was doing pretty well, killing a large number of the strange things coming out of my 4 by4 stairwell which i had managed to dig a largish space around before my miners all ran off. there were things that were trying to freeze my soldiers, but they were dying fast enough that the soldiers were recovering. i had 2 groups of soldiers that i figured i would cycle, with all masterwork armor and weapons in steel, i got empatent and didnt have the adamantium ready yet.
then something called a graphite man came climbing up. he discharged a cloud of cavein dust in all directions that knocked everything silly. two of my dwarves fell to their deaths straight down all the stairwells and into hell itself, and the others all went everywhere along with whatever demons they happened to be fighting. fps was slow enough that i could see the cloud forming, and tried to make my dwarves run away but they were too damned homocidal to heed my futile cries and i didnt think of turning them back into civilians.
another horde of demons came out and began brutalizing all the unconcious dwarves, pretty much wiping out my first squad of 10. last thing i remember looking at before the game crashed due to pure awesome insanity was the splattered corpses of the two dwarves who had fallen into hell, aparently surrounded by little crouds of the easier to kill demons (one eyed three tentacled things made of shadows or somesuch oddness). really looked like their bodies were getting swarmed and devoured.
so, from that experience you could probably hold back hell for a quite a while and maybe wipe out the first massive wave. unless you get something insane from your worldgen coming out, and depending on the aparently random number of demons that spawn when you crack it open. dropping water and magma down the hole to make masses of obsidian would probably be the easiest way to go, and might wipe out a considerable number of the starting wave. modding dwarves to be stronger, bigger and much faster would probably help a lot. and fighting a small stream coming out of a single small entrance would be a lot easier then on the ground in hell itself.
as for walling the place up, hell has more then one z level, and those glowing pits are really really big. plus you cant build constructions right on the edge of the map, and demons would almost certainly come from off map and hang out in the little unwalled area, still destroying your fps.