the only time i seem to get slime staves is when i am farming pumpkin moons, my luck with that is also excessively random, sometimes i only get one, sometimes i get 3 or 4, but most of the time, i get none.
i've had pretty good luck with pally hammers as a collective whole, i had set up an ectoplasm farming station in my dungeon (it is not an afk farm, i'd die if i went afk) and i end up getting quite a lot of paladins to spawn, now if only i could get a dungeon that actually give up the ragged casters so i can pick up some spectre staves xD
the over all longest farm time for me would probably be the ice key mold, since they have become a thing, i've only ever gotten 2 of them myself. i usually have to trade stuff with my brother to get them since he has better luck than i do.