CharlieIAm |
Well, for a Gremlin the obvious choice for Class Levels is Rogue, and if you want to hit the party with the somewhat unexpected there are plenty of interesting Archetypes you can flavor him with:
If you want to go with his strengths, make him a Trapsmith and let his calling card be the nasty surprises he leaves in places he figures the party's going to be.
If you want him to be a versatile threat, Eldritch Scoundrel gives him some Wizarding to add to his inherent abilities, though it does affect his Sneak Attack and to an extent his signature Trap stuff (and after four Rogue levels he could qualify for Arcane Trickster); alternately he could go with Counterfeit Mage and push Use Magic Device, though again it subs out Trapfinding. A third approach along these lines is to go with Underground Chemist and hit the party with alchemy (of course, you could also go with straight up Alchemist - Trap Breaker fits).
If you want to go with the theme of the AP, make him a Numerian Scavenger and equipped with enough Timeworn but nasty goodies to be a serious nuisance (admittedly being Tiny really works against him here - alternatively he'd eventually be a pretty terrifying Gunslinger with his stats, and being able to craft his own stuff helps some with the Size issue).
Quantum Steve |
Adding class levels to monsters is detailed here:
http://paizo.com/pathfinderRPG/prd/bestiary/monsterAdvancement.html#adding- class-levels
Basically you just adjust their ability scores by +4, +4, +2, +2, +0, and –2, assigned however you want. Then you just add class levels and give him equipment and treasure equal to a Heroic NPC of his final CR. (NPC Gear Table)
Class levels should add to his CR one-to-one.
His racial hit dice should also be accounted for when assigning feats and ability increases.
So, he should get a second feat a three total HD or Swashbuckler 2 and an ability score increase at four total HD or Swashbuckler 3 and so on.