I'll go ahead and say this to clear things up, it works way better to be in a "purity" area right beside hallow, i actually achieved this by digging out a 160ish wide by maybe 50-70 tall area, making rows of pure blocks on one side and pearl of the other, i used a 3block gaps (filled in with wood platforms) to switch between them. For ease i used alternating actuators on all the blocks (to drop everything to the bottom most row) went and filled in all the area below this farm 30 blocks deep to foce spawns inside.
Then walked around just high enough to see the lowest level. (Also IMPORTANT to make sure rows are 4 or 5 tall as they require that hight to teleport around) then all you ned to do is walk around in the purity side and find where hollow slimes and bats don't appear, inch yourself over and find that block, mark it and stay on the pure side. Helps to block the row near yourself and use a tile phasing weapon like the death sickle.
This works better because the elementals you want have a better chance to spawn with the purity mobs then hallow. As well they spawn on hallow blocks that fit their 4tall air requirements. I recently heard that lizhard bricks might increase overall spawn rates and caps not sure how that would work in this setup, but it may very well help.
I'll be honest with you, i probably killed 5,000 b4 i got one, but i did that on a device (in the latest update) with only 470ish mb of ram. So crashes may have cost me a few, and it took forever until i finally said
-it and built the full farm.
