Yeah... Sure... More light for a night cityscape...
Better wait for a lightning storm and hope the lightning strikes right when you shoot the picture cause a flash that could light up an entire cityscape would be one heck of a monster.
On a more serious note: Bring a tripod.
My go on it would probably to stop down the lens aperture 2 or 3 stops from wide open (lenses are usually sharpest in that area), keep the ISO low and use a slow shutter speed.
Depth of Field (DOF) is probably not going to be much of an issue if you're standing about 8 or more meters from your subject (which for a cityscape you'll probably be).
You won't need a high ISO if you're going to be using longer shutter speeds. Higher ISO will just introduce noise which you'd be better to avoid unless you really need to get a shorter shutter speed.
Determining the shutter speed can be as simple as just trying it a couple of times. It's quite easy to get the hang of it.
One thing to note is that with the longer shutter speed all moving lights (like cars) will become lines. This can be nice, but if you'd like to avoid this then this is the part where you do increase the ISO so you can use a shorter shutter speed.
Just play with it a bit, it's not too hard and you don't have to be fast because your subject probably won't be walking away.
