I've been doing a fair bit of testing on mob armor over the past month. I've been using pretty much the same method as Lactose, although I use imp. Thunder Clap (which has a fixed damage amount).
To make things easier, I wrote a simple addon that watches for TC hitting my target, calculates how much it should have hit for before armor reduction, and outputs into a chat window the range of armor values that could have produced the observed hit (taking into account any armor debuffs of course). My estimates were then taken from the intersection of all the ranges that I recorded. From the size of the ranges, the accuracy of the results should be about +/- 30.
So far, I've managed to get results for almost every trash mob and boss in SSC, TK, BT, and Hyjal. Some of the values are different from the ones Lactose obtained. I'll just list the boss armor values, because those are the useful ones
.
Gruul's Lair:
High King Maulgar: 7700
Gruul the Dragonkiller: 7700
Magtheridon's Lair:
Magtheridon: 7700
Zul'Aman:
Nalorakk: 7700
Akil'zon: 7700
Jan'alai: 7700
Halazzi: 7700
Hex Lord Malacrass: 6200
Zul'jin: 7700
Serpentshrine Cavern:
Hydross the Unstable: 7700
The Lurker Below: 7700
Leotheras the Blind: 7700
Fathom-Lord Karathress 6200
Morogrim Tidewalker: 7700
Lady Vashj: 6200
Tempest Keep:
Void Reaver: 8800
High Astromancer Solarian: 6200
Al'ar: 7700
Kael'thas Sunstrider: 6200
Hyjal Summit:
Rage Winterchill: 6200
Anetheron: 6200
Kaz'rogal: 6200
Azgalor: 6200
Archimonde: 6200
Black Temple:
High Warlord Naj'entus: 7700
Supremus: 7700
Shade of Akama: 7700
Teron Gorefiend: 6200
Gurtogg Bloodboil: 7700
Reliquary of Souls:
- Essence of Suffering: 0
- Essence of Desire: 7700
- Essence of Anger: 7700
Mother Shahraz: 6200
Illidari Council:
- Gathios the Shatterer: 6200
Illidan Stormrage: 7700
Sunwell Plateau:
Kalecgos (Dragon): 6200
- Sathrovarr the Corruptor: 6200
Brutallus: 7700
Felmyst: 6200
The Eredar Twins:
- Grand Warlock Alythess: 6200
- Lady Sacrolash: 6200
M'uru / Entropius: 7700
- Shadowsword Fury Mage: 5700
- Shadowsword Berserker: 7100
- Void Sentinel: 6800
Notice that all but 2 of the bosses have either 6200 or 7700 armor. A similar pattern occurs in the trash mobs: 5475/5700/5950 or 6800/7100/7400 for level 70/71/72 mobs. In terms of percentage reduction, these values result in either 34.15% or 39.15% armor reduction vs attackers of the same level as the mob.