For a multi-GM campaign I'm doing in the Pathfinder, I need to create a NPC. He's a blue dragon. He's posing as a human senator, and causing all sorts of sneaky troubles. Here's my question: do I just use the dragon template? Do I give him class levels? How can I make him a challenge, well connected, a problem as a background force, not just a physical challenge. I'm looking at a CR of around 16-18 because. Well. He may go head to head with the other GM's boss. I have a fairly good idea of him my head, but I can't make him too hard or too easy.
When adding class levels of a class that synergizes well with the monster's natural abilities, its challenge rating increases by one per level added. For other classes, CR increases by one for every two levels added. As with all things challenge rating-related, it's important to look at the end result and compare it to similarly CR'd things (for example, by the general guidelines outlined here you could add 20 wizard class levels to something like an awakened wolf and it would only add 10 CR, when it ought to add something like 18-20).
Background Force. The easiest way to portray a 'connected' character is through GM fiat and skill checks. Having high Bluff, Diplomacy, Sense Motive, via charisma, skill ranks, magical items, or a workaround for those involving high-level Enchantment use (low level doesn't last long enough or have permanent effects), and having a coterie of minions or off-screen capabilities (sabotage, information gathering, assassination, allied organizations, prepared magic items) to show how those skills have been used, portrays this. Ergo, your dragon needs these things.
CR 16-18. CR is a lot of things, but also a measure of the challenge of 'defeating' a foe by a party of four adventurers of level=CR. Ergo, a CR 16-18 foe needs to be a challenge for a party of level 16-18 adventurers. I have no idea what level of Optimization your party is - a wizard, a cleric, a druid, and a synthesist summoner are a lot tougher than a fighter, healbot cleric, dagger rogue and fireball wizard. I'll assume the latter. This is not, by the way, the investigation portion - that constitutes a separate encounter. This is purely teeth + claws.
The Old Blue Dragon is base CR 16. Why should you go with the base 'age-based' Chassis instead of a more individualized class based mix-up? The simple fact is, Dragons hate class levels. Under the 3.5e CR system, i'd rule that all class levels for dragons are non-associated, their huge HD and LA means advancing as a caster sucks for them, and there are very few classes that complement their natural weapons/breath weapons/spells fighting style. This would allow you to take 2 class levels for each CR added to the dragon, making a Young Adult Blue Dragon Sorcerer 14, for example, at CR 16. In PF, it's not quite so clear cut.
Where I hesitate to simply say 'slap a bunch of class levels on, eyeball it, and have fun!' is that you are apparently planning to have NPC on NPC combat with 'another GM' with it. That means the Fiat should be kept to a minimum. I'll also take a second to go aside and tell you why I think that is a bad idea.
Aside: Why Blue on Blue is a Bad Idea.
NPC on NPC combat can seem like a good idea, especially if you want to show how badass one of the NPCs is right before the party has to take them on (see: the Worf Effect). And it can! It totally can. Rolling the dice can show the PCs that you aren't cheating and that everything in this world follows the same rules they do. NPC on NPC can show the relative power level of new foes, or provide an amazing setpiece for you to describe. But both at the same time leads to a horrible result. The amount of screentime spent on you (the GM, describing both combatants and rolling for both combatants) passes the magic mark where the PCs feel uninvolved and stop paying attention. Spotlight time and agency are a hell of a drug - when players get used to being able to do stuff and have the spotlight on them, taking it away for too long is the absolutely worst thing you can do. You'll lose player interest faster than you can say 'cheese crackers', unless you've set it up with a level of skill that is quite frankly beyond 99% of all GMs. Avoid it, or fudge it. Describe it swiftly, with no dice rolls involved, or don't do it at all, as doing it and maintaining player investment is devilishly difficult.
GM on GM in some ways sounds worse, as you have two NPCs crafted to both bedevil the party, and you can't fudge things, and it has all the problems of the above example. If it's a genuine contest, fudging things is essentially cheating. I suggest if two PC 'big bosses' are fighting, the GMs meet and decide what will happen ahead of time.
But regardless of that. In addition to dragons not liking class levels much, the basic Old Blue Dragon chassis offers some quite fine inducements. The high SR means it won't just go down to a barrage of magic, the AC is relatively decent even for level 16, the spells provide buffs and sneakiness, and mostly, it scales very well with the addition of gear (barding, mind blank, save boosters, physical stat boosters) buffs from an allied caster (cleric or level 13+ wizard or sorc), and tactics. A lot of GMs have dragons land and confront a party with poor full attack routines and hope for victory. That is not the optimal way to use a dragon. A dragon is best used with the feat Flyby Attack, and with 3.5e backwards-compatibility, feats like Empower Breath and Strafing Breath to spit lightning all over PCs from the wing.
A first encounter with a surprised, unprepared dragon who then manages to escape and come back later, buffed (mage armour, haste(note the speed increase), +str/+con, protection from Good, protection from [Element]), wearing barding, with mercenary archers on his back spitting arrows, strafing the party and attacking from the sky, is a much harder encounter, pushing the CR up considerably. It's also a lot more interesting, especially in a varied terrain like a town leading to attempts to grapple the dragon while jumping from rooftops, hiding in building after building as passes by the dragon demolish them with bull rushes and Vital Strike Power Attacking Claw attacks (note Flyby Attack allows a Standard Action, meaning Vital Strike can be used with it.
Spending your CR on the chassis allows you to ratchet it up or down as needs be. No class levels will help you much, even stuff like a Fighter dip to get Full Plate Barding proficiency, or a Rogue dip for the Thug archetype fear ability elicits only a solid 'meh'. Just play to his draconic strengths, use minions to get buffs/distract the PCs, and don't let him get locked down on the ground for no reason, and he'll be a memorable fight.