Assigning CR is more art than science.
See the rules at the Adding Class Levels subsection of the "Monster Advancement" section of the Bestiary.
I'd say that a medusa fills the "special" monster role. In that role, antipaladin is not a "Key" class.
So, under the above rules, every two levels of antipaladin increases the CR of a medusa by 1 until class levels exceeds the monster's CR, then CR increases by 1 per level. A medusa is a CR7 creature, so a medusa antipaladin 10 would be CR 14.
That said, I usually take that as a baseline, build the monster, then check the final monster's stats and abilities against Table 1-1: Monster Statistics by CR on p. 291 of the Bestiary. I'll then pick the target CR I'm shooting for and adjust the creature's stats accordingly to bring them in line with the targets on the chart... this could be by adding a template, class feat selections, appropriate magical gear, or calculating in specific abilities and/or spell effects into the creature's stats, and adusting the monster's Tactics section to make note of it. (e.g. if your monster's AC comes out too low, consider bumping it up with shield of faith that the monster casts just prior to combat; if hp are too low, do the same thing with a bear's endurance effect.)
Also keep in mind that CR is a broad guideline of relative power. For example, a shadow is listed as a CR3 creature, making it an appropriate challenge for a party of 3rd-level characters. But if your PCs can't channel positive enrgy and don't have any magic weapons, then this CR-appropriate encounter with a shadow could easily become a TPK. If you're designing monsters for your own group, you know their strengths and weaknesses and can design accordingly.
[EDIT: Ninja'd by Mark Seifter while I was correcting my own error!]
|Mark Seifter Designer|
|1 person marked this as a favorite.|
Technically, "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" which would mean the first 8 levels were half price and the last 2 levels were full price, for a total of 7 + 4 + 2 = 13. That said, Haladir is correct that it's more of an art than a science and is very easy to abuse if you treat it as a science (succubus antipaladin would also get the discount, but a succubus antipaladin 6 is going to be way harder than a usual CR 10, and a succubus antipaladin 11 is supposedly CR 14 but when added as an extra "minion" in even a CR 20+ encounter will likely turn a reasonably challenging encounter into a TPK with aura of vengeance and her high Charisma score). Think carefully before declaring any class that's adding an ability score bonus the monster is very good at directly into important stuff "non-associated" even if the book says it is.