When you add character levels to something like say a Shadow Demon, does it's CR increase by 1 per character level? So a Lvl 6 Cleric Shadow Demon would be a CR 13?
First off, determine the monsters "role" by checking here:
http://paizo.com/pathfinderRPG/prd/monsters/monsterRoles.html
Shadow demons are a skill based monster, because they rely on stealth to a large extent.
So, if you were to add rogue levels to a shadow demon, it would be +1 CR for every level added, however cleric is considered less beneficial, so the CR increase by 1/2 for every level until the number of cleric levels equals 7 (the original CR), then it goes to a 1:1 ratio.
A shadow demon with 6 levels of cleric would be a CR 10 creature.
However, this formula is more of a guide then a rule. The shadow demon gets its stealth from the shadow blend ability, and being invisible and incorporeal makes a great platform for spell casting and other class abilities.
My thought would be to bump the CR up another +2 if the creature will be used as sort of a boss monster, but leave it as is, or even bump it down a little if the party is equal or greater in level to the creature.
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.
It's much safer to eyeball it, or else you'll get some sort of level 20 wizard monstrosity facing your poor level 12 PCs.