I am AA 15 (about to be 16 as you will see). First time I went into Ghosts of Perdition few weeks ago was a disaster. Everyone died on wheep brothers, recalled, re-entered, then barely defeated the already-weakened wheep brothers with several more deaths; I recall expending quite a few Aerratikos cookies in that battle. Shrined up and went after Cholthuzz, but once he removed our buffs it was all over. Not sure about the others, but I certainly was unprepared. Normally I rely on Imbue Force (or Force Burst) to hit incorporeal undead; without Imbue Force, whatever bow I was using did not do much good against a lawful good incorporeal.
This time was different. Very first thing I did after that inglorious fight was buy Ghost touch longbow of Maiming on AH. Soon after I lucked out in Shroud and pulled Holy burst bow of Greater undead bane. Holy burst part is irrelevant to Cholthuzz, but the other part is not. I stocked up on Anarchic arrows, pulled Shards of Light out of storage, and finally got a Scarab of spell absorption. Not sure if that made any difference.
Jessemina, Alderich and I each got a cleric hireling, so we were two rangers and FOUR healers -- nobody over level 17. All trash mobs got disposed of quickly, as I expected. Wheep brothers went down with just one death -- and I could see my Shards of Light were doing massive damage. So my plan for Cholthuzz was: equip Greater undead bane bow and Shards of Light (same as for wheeps); when they run out switch to Ghost touch maiming bow and Anarchic arrows -- and have quiver with all other Anarchic arrows equipped, so I automatically switch from +2 to +3, etc. as I use them up.
It almost worked. We did not even try for the optionals -- just crowded around Cholthuzz and let each ice floor break and send us down level after level leaving trash mobs behind. Unfortunately you can't do that with last level. I knew we were in trouble when two wheeps aggroed on me, and I had to choose between defending myself and putting arrows into Cholthuzz. I frantically drank several Restoration potions, but it was a losing race. Everyone died with Cholthuzz around 15% health.
Jessemina and Aderich recalled, I kept the instance open. On the way back from Necropolis to the quest Jessemina, of all the luck, ran into Orchard beholder! (She gave it a wide berth.) Once they were in, I recalled and re-entered. That's when I realized I never put on my Scarab of spell absorption. Well, I fixed that oversight, but have no idea if it made a difference or not. Within seconds after engaging Cholthuzz I had 4 neg levels -- granted, one was due to Anarchic arrows, -- and few seconds after that golden chalice appeared and the quest was over. "You have enough experience for next level." Yay! My scarab lost six charges.