What I picked up in 20 Nihlathak runs today (using a CL/Nature's Peace Sorc):
1 - the Halls of Pain are set-up much like the Durance Level 2. Follow the left wall, or go clockwise around the outside, and you'll usually find the stairs rather quickly. The only really dangerous creatures in the Halls of Pain seem to be the Frenzytaurs. Once you get to the Halls of Vaught, nearly everything is dangerous.
2 - Even if your merc does no damage AND you use Nature's Peace, you will still get CE'd. Nihlathak does the Shenk-like whip em into a frenzy thing that makes his minions blow up. If they do so before you kill them, they leave a body.
3 - FE and Cursed are BAD BAD mods on Nihlathak. His Corpse Explosion damage appears to be about 1k with 2 people in the game (not sure if the #players affects this) normally, but with FE, it is significantly more. Amp... well, that goes without saying.
4 - At first I debated whether to use ES or not. There is no doubt in my mind now. USE IT. There are too many things that hit too hard down there, from CE to the Charging Re-Animated Dead. There are also Frenzytaurs in the Halls of Pain. The near-instant emptying of your mana pool when the Vipers hit you is insignificant compared to... well... dying.
5 - Sol Runes are your friend. Load up with em. Put em in every socket except your weapon. I currently have damage reduced by 39, and it nearly negates the physical damage part of the viper's bugged attack. It still makes the machine-gun sound and drains your mana pool, but your life drops very little or not at all.
6 - Nihlathak can be Lightning immune. Your options are pretty limited with a Lightning sorc if he is, especially if you use a very low damage Insight weapon. Of 20 runs, I only actually killed him on 17 of them, and on two of those I had to switch to a different character to kill him.