Okay so i was exploring the map like you do, and i was heading towards "The Devils Lair" but not on the actual mission. On the way there is a big fight between the Fallen and the Hive. After killing them all and you turn right is a hallway with green glowing crystals and if you go towards it you see its a tunnel and it goes pretty far down until you reach the the Hive enemies. There level is [??] and its all in red, I have some pretty powerful weapons and heavy weapons, no matter what you throw at them or shoot at them, you wont even take 1hp off them. There are at least 20 of them (two being wizards) i tried all my heavy ammo and all i managed to finally take of him was 1hp to his shield and thats it. It's crazy! But it was quite amusing. After allowing myself to be killed by one of these immortal crazy hive dudes, his level finally came up and he's a level 20, but even still... it's impossible to take even the smalliest of health off them. Just thought you should know about this bug, stay well away from this tunnel! :)
That doesn't make them immortal.