I was trying to complete a patrol on Venus called "Persistent Threat" and every time I came across an invincible enemy in the exact same spot. One time it was a "devoured minotaur" (with a slap rifle) and another time it was a "tortured captain". In both cases the enemies were in the exact same spot of a big cave, where a bunch of enemies were fighting amongst themselves. Both guys were partially invisible, and ALL attacks were immune/resist. I don't think the level was labeled as "??" as some other invincible guys are. I tried everything - supers, grenades, primary/secondary/heavy weapons of all damage types. I suppose the only thing I didn't try was melee. I also tried waiting to attack until they were taking fire from other minions. No luck. The enemies make that patrol (Persistent Threat) un-completable. I wasted hours trying. So annoying! I've come across a couple other invincible characters in other missions but they seemed to be intentionally placed to make areas of the map inaccessible or to guard goodies, etc. in this case they're just making this stupid patrol un-completable.