Oh, another man asks this. Another one just asking.
So, my friend, you want to conquer Hell?
That's, technically impossible.
But we're dwarves, we take technicalities and dump them in the magma. What you need, is for once, a cave-in gun ready to fire at your breach. Perfectly, this design would be placed above the bottomless pit, so it never clogs. And I do mean the obsidian generator. If that cannot be achieved, your second best hope is to drop in an absurd amount of bred, trained-to-war cats (modding is extensive. You're in Hell, damnit.) and saying your prayers to Armok, dropping in a few legendary adamantine-clad dwarves, and hope it works. And finally, there's the last approach, the super-huge DemonSmash hallway. Perfectly it'll take up to a whole z-level of a 3x3 embark and will be filled all with upright spikes with metals no worse than steel installed. All of the spikes are connected to a lever. You pull the lever until they reach your bait (dwarf or otherwise). If the initial wave dies off - you're good, you can unseal yourself if you want, but the demons won't try to rush you so much. Their minds have adjusted, and they think - rightfully - that you're a force that can stop demonkind right in it's tracks, even the worst of them. I suggest having a good military trained before you get out of your bunker, they still can be aggressive.
Oh, and remember to floor the glowing pits. It's the most important thing, and you must remember to do it.