Well, I think with that question you have a firm grip on making dungeons into grinders for fun and profit. The only suggestion I have is that with the new soul sand water mechanic, you can actually have the skellies making a trip up to a nice collection point on the surface rather than having to go underground to get at them. Since spawning occurs in a cylinder, not a sphere, this design works great. I have a zombie and skeleton spawner that drop right into the same collection spot without having to get near them, level-wise. As a reminder, skeletons cannot drown.