Immortal* Expedition mobs

From time to time, I've encountered almost immortal mobs during the Expedition trial. I can't say it's a bug, but they're so resilient that you just can't kill them.

I can't remember their exact name — they were small, black, and fast, kind of monkey-like. They had life regeneration as their only listed modifier, but it felt like they had extremely high resistances, armor, and immunity to bleed. I remember struggling to kill them back in version 0.1 with my Ice Monk. Now I'm playing a Blood Mage with bleeding.

Just to clarify that it's not a damage issue — I was able to complete the map with them chasing me the entire time. But to kill them - I had to explode them with herald while they were surrendered by a ton of mobs.