I heard that the slime staff is the rarest item in terraria. And I wanted it so bad that I decided to farm it but I don't exactly know how to farm it.
The Terraria wiki's Slime Staff page states:
An easy way to get this staff is to use Slime Statues, with a thin layer of lava.
The Slime Staff is only dropped by Slimes and Slimed Zombies, with a low 0.01% drop rate. With this in mind, it may take time, but the Staff should eventually be found by using the aforementioned Statue method.