I know an 11th level cleric's magic is piss poor against a party that could take out an epic level CR, but I'm still interested in an explanation of the pertinent game mechanic.
Associated Class Levels
Class levels that increase a monster’s existing strengths are known as associated class levels. Each associated class level a monster has increases its CR by 1.
Barbarian, fighter, paladin, and ranger are associated classes for a creature that relies on its fighting ability.
Rogue and ranger are associated classes for a creature that relies on stealth to surprise its foes, or on skill use to give itself an advantage.
A spellcasting class is an associated class for a creature that already has the ability to cast spells as a character of the class in question, since the monster’s levels in the spellcasting class stack with its innate spellcasting ability.
Nonassociated Class Levels
If you add a class level that doesn’t directly play to a creature’s strength the class level is considered nonassociated, and things get a little more complicated. Adding a nonassociated class level to a monster increases its CR by 1/2 per level until one of its nonassociated class levels equals its original Hit Dice. At that point, each additional level of the same class or a similar one is considered associated and increases the monster’s CR by 1.
Levels in NPC classes are always treated as nonassociated.
pawsplay said:
Looking it, my thought was, "How is cleric non-associated for a strong melee character with a Wis bonus?"
Because the person assigning the CR felt the spells the EG-C will bring to the battle won't be significant to warrant the levels being associated?
The spells can hurt, but the EG has a lot of very good options for every round of combat. And that is why nonassociated levels are a big factor of classed monster CR. Most of the times a monster's natural abilities are more dangerous than whatever ability the character class has bestowed.
The EG-C may have some good potential, but any round he casts a cleric spell is a round he is not attacking with his ungodly BAB. Even in worse case scenario he has advanced warning of the party and pulls off the “50’ tall and farting fire” combo of Righteous Might, Divine power and Divine favor, a single targeted greater dispel slags all his buffs. And I would partially fault the party for dragging ass for giving the EG-C the time to buff up in such a manner...
It did look cheesy adding the 11th level of cleric and not bumping CR. I would definitely say round up, not down, the amount of non associated level in that situation. {CR21, not 20]
I have not play tested high level play, but perhaps treating the cleric levels as semi nonassociated [4 levels / 3CR ] might make a better approximation of CR, something around CR 23.
Overall, my gut says someone needs to make sure whoever assigned that CR gets the “CR is an art, not a science” memo.