The rules on adding levels to monsters are a little complex but I do broadly understand the "key" and non key classes as labelled and how that works with CR. And the logic makes sense
However the Occult classes don't have these applied.
So it is not clear to me how to work out the CR when adding occult classes to monsters. Hero lab treats them all as non-key
But I just tried it and the output just looks rather frightening for the CR.
I think something like Mesmerist (and Medium?) will be key for whatever a Sorcerer is key for. Such as a Nymph Mesmerist
But the others are trickier as there aren't many intelligence based monsters.
And Kineticist is an odd one again. Being dex and con based makes many monstrous creatures a very potent chassis. So should it be "key" for something like a Giant. Say I wanted a Fire Giant Pyrokineticist?
It's not all about ability scores. A fire giant is an effective melee combatant, with a greatsword which does ~25 damage per attack (~34 with power attack) for three attacks, or a thrown rock which does ~23 damage for one attack. A fire giant pyrokineticist has alternate options (not ones which stack) which do a lot less damage until quite high level, and the CR should barely shift for the pyro levels.
The amount of CR you should add for class levels is an art not a science and the +1/2 or +1 CR depending on whether the class is 'key' is a rule of thumb only.
Perhaps the fire giant was a bad example. Although I was thinking of one who was a kinetic knight / used the melee options
What about a monster that is a more Dex based melee character. I know there aren’t many but as monsters they also have high con. Although not a melee character a nymph has high Dex and con. You use the monster class level rules to give +4 to Dex and con and boosts from class levels
Add in the kinetic melee options and take some burn or do some charging up (plus the elemental overflow) and you get around +8 on both key stats and a shocking amount of damage. Principle applies to other Dex based monsters with higher underlying BAB than fey - for example I think there are some demons and devils that aren’t pure brute strength. Like a succubus ...
Generally speaking, a character class is considered "key" if the class abilities stack or directly compliment with the monster abilities/role in some fashion.
A nymph, who casts spells as a 7th level druid, would count druid (and only druid, unless changing the nymph spellcasting to a different class like hunter) as a "key" class for CR. Since a nymph would be considered a "spell" role encounter, any other class would not be considered "key;" pretty much for the same reason adding levels in other classes doesn't usually make PC spellcasters a lot more "effective" (compared to advancing in the primary class). Sure, they can do things other than cast spells, but they will probably not do them as well as an equal level PC single-classed in the class that the spellcaster is taking those additional levels in; and they give up increasing their spellcasting.
The "combat" role can be either Str-based (melee brute) or Dex-based (finesse melee or ranged). Granted, there are more Str-based combat role monsters, but that doesn't change the role that the monster is filling.
Prestige classes can make this trickier, but a good rule of thumb for prestige classes is to count a full BAB PrC as a "combat" role class, any PrC with 6+ skill ranks as a "skill" class, and a PrC that increases spellcasting as a "spell" class. Yes, that means that a prestige class can count as "key" for multiple monster roles.
The issue is how to categorize the occult classes for CR advancement. Personally, considering that kineticist abilities are so different from most monster abilities, I'd say that kineticist levels should be in the same category as monk and paladin levels (since kineticist is 3/4 BAB, it doesn't directly improve combat ability like a full BAB class would, it has 4 skill ranks per level, and monster HD/abilities don't directly improve kineticist abilities). Medium would probably be considered the same: it's a 3/4 BAB class, 4 skill ranks per level and not Int-based casting, and for a spellcasting role monster, the medium spells for channeling an archmage or hierophant spirit would be counted separately from monster spellcasting (just like a sorcerer/medium or cleric/medium would count the spell progression separately). Mesmerist and occultist should probably be counted as "skill" role classes like bard and rogue; the occultist has 4 skill ranks per level, but it's an Int-based caster and has a bunch of useful class skills. Psychic is counted as a "spell" role. Spiritualist should probably also be not counted as a "key" role.
OK, a nymph. Suppose they're adding 7 levels of wood/earth kineticist. Not kinetic knight because nymphs don't do heavy armor.
With 3 points of burn on her defence she has +3 natural armor, 15% chance of ignoring crits/sneak attack, +2 attack and +4 damage with kinetic blasts, and +2 dex & con.
In total that makes 15d8+90 hp -45 effectively from burn (average 112), AC 32, BAB +9. Gather power and infusion spec let her use 2 burn blasts without cost (except a composite blast which still costs 1). Attack is +20 with ranged, +20/+15 with melee on a full attack. A simple blast does 4d6+15 damage, autumn blast does 8d6+19. Without gather power only a simple kinetic blade avoids burn, but a standard action kinetic whip will sometimes be useful for reach tactics. She has 1 point of burn in her internal buffer too.
What CR does this look like? On AC it's about CR 11, attack is about CR 12, HP is about CR 9. If allowed to prebuff with her druid spells her stats will be better, but prebuffs usually aren't included in the stats I'm using for comparison. It looks like the +1/2 CR per level was about right.
& the trouble with doing stuff like this in your head is that it's easy to work off a previous version - the stats there are using weapon focus (kinetic blast) instead of toughness. Using toughness instead makes for slightly more even CR-equivalents.