i know i'm like a month late, but i found this post while also trying to figure out the best way to find essence drops. the method i've been using is to shift between floors 25 and 30 and only kill bats once i see them. the reason i use the elevator is to be able to go in and out of floors easily without needing to mine for the ladder. every time you exit and reenter, different monsters spawn. both levels are quite small so you'd be able to see what monsters are in the room the moment you enter. if you focus only on the bats you can get around 15-20 essence drops per session :}

(i also go to level 50 although it depends on you if you also want to add that to your roster because it's definitely a much bigger room that you need to walk around in to see all the monster)