I have figured out a time-saving trick for aquifers which can avoid a whole heap of suspensions when using pumps, it only works to get through a single aquifer layer, but it is absolutely perfect for the final level of twin slit, and also works very well for normal 1-z aquifers if you don't feel like doing a cave-in. Essentially it allows you to put a dwarf under the aquifer
before you've walled off the layer and that dwarf can establish a drain, drains are immensely time and tedium saving especially when you don't have dfhack `autounsuspend`.
The trick is simply this, dig up/down stairs, NOT ramps, into the aquifer, they must be up/down stairs. This is normal for twin-slit. While a pump is keeping the tile dry, build a hatch on the tile. With the pump still operating you can now designate an up/down staircase under the hatch, a miner can dig the up/down stairs immediately under the hatch and pass through the hatch, 0-2 water will slip through with him and the hatch will then close and prevent any further water from flooding the staircase (at this point the pump can be switched off), allowing the miner to extend the staircase down to the caverns, establishing a cavern drain.
With the cavern drain established, you can then finish walling off the aquifer layer, enjoying 0 suspensions if you channel out the tiles where walls will go, or few suspensions if you dig them. The nice thing about this trick is you don't do anything you wouldn't normally do anyway - it's normal to punch a staircase straight down until you hit a cavern. It's also nice if you have a dreadful layer-cake aquifer where layers of magnetite alternate with layers of conglomerate, using the hatch trick you can nearly instantly bypass non-aquifer layers and re-establish the drain where the aquifer restarts.
Ascii Art Guide:
z=0 z-1
###### ######
#X..X# #X##X#
#X..X# #X##X#
###### ######
Use typical twin-slit, up/down stairs going into the aquifer, floors to build pump on.
z=0 z-1
###### ######
#XPp~# #X##H#
#X..X# #X##X#
###### ######
* Build pump
* Channel out pump inlet tile
* Operate pump
* Build hatch on tile being drained.
z=0 z-1 z-2
###### ######
#XPp~# #X##H# # X
#X..X# #X##X# # #
###### ######
With the pump still operating, designate up/down stairs on level below the hatch. Miner will dig out up/down stairs, and can pass through and do further digging.