Trying to figure out the CR of an advanced monster

[Gannerthehunter]

My friend and I are creating advances monsters to pit in 1-on-1 arena battles. Our monsters are supposed to be between CR 23 and 25, with a plausible back story to explain any alignment variations from the bestiary's.

My creation started life as a Movanic Deva with 20 levels in Paladin that succumbed to temptation and fell, not only as a paladin but an angel as well becoming an anti-Paladin.

The monster advancement rules in the bestiary list paladin, and I'm assuming anti-paladin since it's an alternate class feature of the paladin, as not being a key class for a combat role monster when determining the new CR. Following those rules my monster should be CR 25, 10 from Movanic Deva, and a total of 15 from anti-paladin. My friend disagrees and says for this application it should be key for my monster. Any thoughts, considerations, or comments are welcome,

[WelbyBumpus]

I would vote with your friend, and I would argue that anti-paladin is in fact a key class in this case (or, if not technically a key class, the class levels sufficiently empower the monster to add levels on a 1-for-1 basis).

Note the excessive use of terms like "generally" and "guidelines" in the text here:

Bestiary wrote:

Step 3: Determine CR

Determining the final CR for a creature with class levels requires careful consideration. While adding a class level to a monster that stacks with its existing abilities and role generally adds 1 to its CR for each level taken, adding classes that do not stack is more complicated.

Table: Monsters with Class Levels gives general guidelines regarding which core classes add directly to a monster's abilities based on its role. Classes that are marked “key” generally add 1 to a creature's CR for each level added. Classes marked with a “—” increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added). Creatures that fall into multiple roles treat a class as key if either of its roles treat the class as key. Note that levels in NPC classes are never considered key.

[WelbyBumpus]

It's probably helpful, for this exercise, for you and your friend to agree on whether the advancement rules in the Bestiary are going to strictly apply, or whether they will be just used as guidelines. This seems to be your fundamental disagreement here (you're reading them like strict rules and he's reading them like guidelines).

If the latter, you should find some way to agree with each other upon whether classes will or won't add to CR on a 1-for-1 basis.

[Paul Zagieboylo]

For an academic exercise of this nature, I would probably just rule that all classes are always considered "key" for the purposes of increasing CR. You're just making gladiators from whole cloth, you're not going to take class levels that don't support your base monster's concept. If you were going to do that, you'd start with a different base monster that better supported your class.

The non-key reduction is basically meant to reflect the fact that giving a troll one level in sorcerer simply does not increase his danger potential by 50%. A troll cleric 1 is still pretty much just a CR5 troll. The most annoying thing I can think of that he'd do with his cleric level is to cast shield of faith and maybe divine favor, but it doesn't improve his attacks much, he's even dumber (or clumsier) than a normal troll, and he still gets his face exploded by anything that requires a Reflex save (his Will save isn't that hot either, even as a cleric). Anything else he could conceivably do with his cleric level pales in comparison to his standard full attack. A troll cleric 1 is simply not a CR6 monster. Neither is a green hag barbarian 1. (A troll barbarian 1 sure would be, yikes.) But the entire system sort of breaks down once you have more class levels, which is why it's merely meant to serve as a guideline for a GM who wants to put some neat, quirky monster in the adventure. If you're just doing this to have pit fights, all classes should be considered key to avoid arguments of just this nature.

[Gannerthehunter]

Thanks Welly and Paul for the feedback. You both were able to shed light and perspective on the problem we were facing. We agreed to take your advice Paul and make all class levels 1-to-1 for this and future arena matches we have.