The Dropped By and Drop Rate isn't quite right. Basically what you want to do is kill the non-elites, eventually a huge elite one will spawn. RNG is RNG, could be one up when you get there, could be 15 minutes of running up and down the filth pit killing them all before one spawns. The elites have a chance to drop them. Although once I think I did loot one from a non-elite, but it could have been a a leftover from a missed AoE loot from when 2 elites were up, who knows.