Jay Wilson stated in his interview that the Uber versions of the bosses would not simply be increased hp/damage versions of the existing bosses, but that they would not create totally new bosses either.
So that means primarily some new procs and fighting strategy.
What are the differences between the Inferno bosses and the Uber bosses?
What are good strategies for facing off against the pairs?
Best Answer
So the 3 sets of bosses are (Source):
As far as I know, the only one with a new ability is Magda with spawning arcane sentries. Leoric still has his charge up 4 hit combo swing, but the animation seems to be buggy. He charges up, and then walks very slowly and kills everything within 8-10 yards. Other than that, every other boss just seems to have a lot more life.
For strategies, I ran these bosses (as a monk) on MP4 with another monk, a WD, and a DH.
If you intend to fight all 3 sets of bosses in a single game, you may want to plan around killing an elite pack between sets to refresh the timer on your NV 5 stack.
Zultan Kulle and Siegebreaker
I tanked Siegebreaker away from the rest of my group as they zerged Zultan Kulle down. The WD was key, as he could chain hex with his Gargantuan and keep Zultan Kulle from doing anything. Once Zultan Kulle went down, we would just kill Siegebreaker. Honestly, these two seemed to be the easiest of the three for my group. Siegebreaker has reflect damage, so preparing with some LoH is a big asset.
Magda and Leoric
This fight is a little tricky. Our first attempt, Magda shielded at 75% and even after we killed her adds, she never dropped her shield (Looks like Leoric must be killed first). Regardless, you will probably want to focus King Leoric down anyways, as he summons a lot of skeletons and can kill your ranged DPS if they're not careful. His charge up swing MUST be avoided, and it cannot be interrupted. Whenever I saw it coming, I would get out immediately, or hit Serenity.
Magda will be firing off insects the whole time, which are pretty hard to see due to the dark blue background, and placing arcane sentries. (Not sure why Blizzard designed these rooms to be like the shadow phase for Diablo).
Ghom and Rakanoth
This fight took a little bit to get used to, but the key is to separate the two bosses so the rest of your group isn't standing in poison. I tanked Ghom in a corner and with enough LoH, Mantra of Healing (Time of Need rune) spam, and the occasional blind/serenity/breath of heaven/health pot, he couldn't take me under half hp. The other monk sat on Rakanoth and they eventually took him down. I'll have to verify if hex works on Rakanoth as I wasn't able to see what happened to him during the fights.
Crowd Control
Since Blizzard buffed CC to work on bosses, you will want items with chance to stun on hit and probably at least one CC ability if possible. The abilities we ran were:
Other gear
Outside of CC, you want to just be able to live through these fights. That means don't sacrifice too much life for damage. I regemmed my gear to go from 23k hp to 41k hp, dropping about ~15k damage. I also picked up 800 LoH, which was extremely effective.
Edit: There appears to be an enrage timer, however I'm not sure what the time is. I've heard reports of between 10 min-15 min, so you will have to meet a certain dps break point. Survival is key, as no matter what your damage sheet says, you do 0 dps when you're dead.