Ok, I have an idea, an adaptation of the pancake batter thing or whatever it's called:
Basically, fit in as many pumps as possible so that the room fills as quickly as possible.
So. Z-levels go like this:
0) Image, with all pumps pumping in water from a river, brook, HUGE aqueduct - possibly at a meeting of multiple rivers? An aquifer?
1) Open Space
2) Image, with all pumps pumping in magma. This could come from magma seas, which continually replenish from semi-molten rock.
3-X) Open Space, as needed.
X+1) Demons path through here.
|LLLL|
| |
|W W|
| |
| |
----------- ------------
DEMONS ->
_________________________
Hook the whole thing up to a repeater so that both bridges open the instant both rooms are filled up. Possibly a pressure plate in one of the rooms? Anyway, as in the pancake batter one, both liquids drop as a 'sheet, the lava lands first then is nearly immediately turned to obsidian. Then, the demons fly up over the first layer, and are encased in the second. As soon as this reaches the top the demons will be blocked out while you mine through all the obsidian you have created.
The best thing would be to make a massively long 1-tile corridor that uses this method... This would ensure that the demons pathed through it.
Hope that makes sense
EDIT: Made the image a bit big, cba to change and reupload it so I spoilered it.