There is nothing that says you have to do anything.
From what I have read of your post, you have the demons contained outside of your fortress. You are under no obligation to defeat them now or ever. If you do decide to defeat them, then you can do so later rather than now. I would personally set up an area just inside your containment perimeter, a simple 1×3 corridor with a door at the far end, and beyond that perhaps a 3×5 area with a support at the far end being the only thing holding up a couple layers of ceiling. Build up a military fielding adamantine or steel axes and silver war hammers, send them into your 1×3 fighting corridor and let the demons in to fight them. Perhaps your 10 or 20 dwarfs will kill all the demons if you trained them up well enough; if not, then they will die and their gear will be dropped in the 1×3 corridor, the demons will break the door behind them, rush into your 3×5 collapsing room, destroy the support and be crushed. Just to be extra safe, on the far size of the collapsing room trap, have an upward floor hatch or something else that building destroyers can't break, to seal off any who happen to get through the collapse. Even if your military squads die, their gear will be sealed away, safe from the cave in and ready to be recovered later. If there are still demons living at the end of this, then do it all again: make another battle corridor, another cave in trap, another wave of military dwarfs, and repeat until you finally overcome the initial wave of demons that spawned upon breaching Hell. Once you have taken care of them, the stragglers that spawn on the edge of the map shouldn't be too much of a concern.