I find it works best of you are on the purity edge of a hallow cavern. (So normal mobs spawn, lots of bats and skeleton archers mostly. If your the right distance away from the hallow the elemental will spawn in the hallow and move towards you off screen.
I did make my farm easier by setting up alternating actuator blocks. (Off/on/off/on) connected to a timer and each with a different color wire connected to levers. (So i can switch on or off independently) this causes all mobs to drop to the lowest level and if desired hoiks can be used to force all mobs into one point. (Lava pits perhaps) as well it makes loot collection much faster.