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I don't have this one yet, but I noticed the mobs have stacked buff called "Filthy" on them. The higher the number of stacks, the larger the mob. The # decreases as you fight them. I found that when I killed mobs that when added together reached 100 of the buff, the elites spawn. I'm not sure what the time limit is though. It could be coincidence, but it happened every time I kept count.