All I know was that everytime I hit it "immune" would pop above his head instead of any numbers and it didn't look like his bar was going down.
Yeah, there are two guys that I've seen that have immunities: one to fire, one to frost. They're both in the bunkers in the Battlefield of Eternity. The one is in the event where you have to click the three crystals and kill the mobs that come out. The last one summons a few demons and one named that is immune to fire. The cold immune one is in the occupied bunker where the named demon is just sitting at the end surrounded by other lesser demons. The first time I encountered the fire guy was on my Monk, and it took forever to kill him since most of my Monk's abilities are fire. I'm pretty sure my follower did about 90% of the damage to him.
I've found some really cool ammys recently. Don't remember the names, but one makes me not only immune to arcane, but arcane actually heals me the other is like a deadmau5 ammy that seems to cause enemies to jump around uncontrollably when I cast bbv.
Yeah, there are amulets that heal you for 25% of the damage that you would have taken for all elemental types, but the arcane one seems to be the most common. I've found the Deadmau5 one, but it seemed a bit weak and the proc didn't seem all that useful for me.
EDIT:
I do agree they should make elements actually do something. Fire could burn, poison could add a DoT, cold could slow, lightning add a shock stat that increased damage taken and holy possibly have chance to blind or something.
That's what I don't understand about this whole elemental affix swap for abilities. People are complaining that they're pigeon-holed into fire because "it's the strongest." My first thought to that was, "No duh?" It may seem blunt, but to me it seems obvious. In all my experience in Blizzard games, abilities that have secondary control capabilities
have weaker damage components. As an example, for the longest time in World of Warcraft, frost Mages were the weakest build in PVE. They were like this because the class had a ton of control through its abilities, and making them as potent as fire or arcane would lead to them being too strong. I think it wasn't until the previous expansion where Blizzard finally tried to address this. Anyway, for me cold abilities tend to slow and/or freeze, and lightning abilities tend to have a chance to stun.
In my own anecdotal experiences, I know that when I removed the Burst rune from Justice on my Crusader, I started noticing my health pool becoming considerably more volatile. Simply putting the rune back kept my health pool at around 100% all the time. Why? Because Burst is a Lightning-based rune that can cause a stun. The stun kept enemies (including elites) from dealing as much damage to me.