In reverse order -- whatever you can kill them with, big enough for you to move around, and persistence and tenacity (and to not go "Ok, it's 1/35 odds, I killed 35, it hasn't dropped yet, it's bugged.")
In short -- kill the enchanted swords that drop it with whatever you've got that does decent/good damage, and have a big enough area in the underground hallow to dodge if you need to.