User blog comment:Kablanthony/Lens,Lens,Lens..../@comment-5686539-20151218194324

In addition to what The Pathogen suggested, if you have access to the dungeon, you could snag one of the many Water Candles that are found within to use in addition to or in place of the Battle Potion.

Also, when you are trying to get them to spawn, I'd recommend staying away from the Corruption/Crimson. I don't remember nor can I say for certain, but you might have better odds for them spawning if you don't have a constant influx of Corruption/Crimson mobs coming for you. Lastly, try during a Blood Moon.

I think that's everything...