Adding class levels to a Rust Devil

[ZanThrax]

If I wanted to add Spell Warrior Skald levels to a Rust Devil

https://aonprd.com/MonsterDisplay.aspx?ItemName=Rust%20Devil%20(Ferrugon)

how many levels should I add to get it to a CR 15? 3 or 6?

And is there a different class / archetype that I should consider to give him some arcane magic (including alter self and/or similar transmutations so he can disguise himself as a human), medium armour and martial weapon proficiencies, and some skill points he can sink into Craft skills?

[Bjørn Røyrvik]

CR increases by 1 for every level you add. It's an imperfect system, but the best a generic rules set can do for now. More important is to know the system and know your players and build according to that. As a Skald you already get arcane magic, including Alter Self.

[ZanThrax]

The reason that I ask is that the rules, such as they are, for adding levels to monsters, talk about creature roles and deciding if the class you're adding is a "Key" class for the role.

https://aonprd.com/Rules.aspx?Name=Adding%20Class%20Levels&Category=Mon ster%20Advancement

The problem being that while I'm pretty sure if I'd consider the Rust Devil a "combat" role enemy, I'm not sure if I'd consider the Spell Warrior to be a skill class like Rogue & Bard or closer to a full martial class; thus the question about counting the levels 1:1 or 1:2 for CR.

And the other part was just that I'm not certain if Spell Warrior is the most effective / interesting way to get the general effect that I'm looking for - giving the devil magical disguise capabilities that doubles as a potential combat boosting spell, the skill ranks to have it be an excellent smith, and the proficiencies to use the weapons and armour that he makes.

[Bjørn Røyrvik]

How about Magus? They get Alter Self, weapon and armor proficiencies and while they may only get 2+Int per level, the ferrugon has an Int of 18, so that means at least 24 skill points to put into Craft by the time you get 2nd level spells. Medium armor is a bit trickier and you have to wait until 7th level to get that unless you want to spend a feat on it and even then you'd still be getting arcane spell failure (though if I were GM I could see my way to reducing it by the highest ASF rating of light armors).

As for the CR thing, I would say something like the Magus would help its smashiness more than something like Skald. However, the Skald is a bard barbarian hybrid so it's intended to be a bit more fighty than pure bard, so by the rules it probably counts as a full level.

I'll reiterate that the important thing is to determine how difficult an opponent is for your group, and CR is a rough guide at best.

[Artofregicide]

I'd argue not key class, 6 levels. Actually, 7 if you want - the half CR levels round down.

Keep in mind your parties overall optimization vs. that of the monster. Don't knock down the gates with an absolute monstrosity vs. pregens, but don't feel limited if your party are shameless optimizers.

[VoodistMonk]

I would also say non-key, and go for 6-7 class levels.

[ZanThrax]

Thanks guys. My players aren't hard-core optimizers, and while they can take most of the content in the AP without much difficulty, but boss fights are challenging as published, so I don't want to go too nuts, but since I'm planning to replace a boss with this guy, I don't want to make him a pushover.

Maybe a 7th level Soul Forger Magus...