Get a pen and paper, make a note of the level number and layout (I do a quick sketch of where the stairs are), and join these to show which staircase leads to which level. I've only done it a couple of times but IME it can be "up and down", but maybe I just didn't find the fastest route.
Alternatively get hold of lots of ?oMM.
Don't underestimate how much food you are going to need - I went in with ~100 blessed lizards and had to make a hasty retreat.
Gate closers: L+ DE Mindcrafter (p20), N= Gnomish Assasin (p20), N= Gnomish Mindcrafter (p23), N= Ratling Ranger (R49)