Monster Advancement

[Trisagon]

Hello everyone! I am trying to stat a CR 15 Succubus but I am facing some problem. While I find it easy to add class levels and adjust the CR, I still have issues while trying to advance monsters with racial HDs. The main problem I faced was the ability score adjustement. Following the official "Monster Advancement" guidelines, I can adjust the ability scores in order to reach the suggested damage and HPs. What happens when I try to advance a monster such as the Succubus, which also has Spell-like abilities? Do I have to reach suggested damage, AC, HPs and DCs or do I just focus on either the combat or the spell-like abilities?
Thank you all in advance

[Dasrak]

The appropriate abilities that a monster should have for a given CR are inherently a subjective call. In fact, you can see a big difference in Paizo's own judgement on this matter by comparing Bestiary 1 and Bestiary 6; in general, the more recent monsters tend to have more influential abilities than ones from earlier books. Given the large leap in CR you're talking about, you should definitely consider adding more abilities to this custom succubus.

As for combat power, you should at least try to stay reasonable close to guidelines. There's no need to be exact, but if you're substantially over or under guidelines in many categories you'll probably want to correct that.

[Trisagon]

I was very confused to see that monster abilities often exceed DC guidelines. I guess I will just take an higher level demon and try to roughly compare the CR.