I'm currently DMing my first Pathfinder game, and I'd like some advice on how to handle the "boss fight" with one of the main antagonists.

Basically, what's going on is that the PCs are in a race over the (rather small) continent with the 3 antagonists, a set of high-level Drow agents to a series of powerful artifacts that, when all are collected, will play a big role in how the campaign will turn out. The PCs are 7th-level (Bard, Fighter, Ranger and Cleric), and the bad guys are a 15th-level Alchemist, 17th-level Rogue/Master Spy, and a 19th-level Cleric/Sorcerer/Mystic Theruge. They already killed the Alchemist, and up next is the Master Spy.

I'm just not sure how to design a fun, interesting encounter that optimizes the Spy's abilities, which are essentially tons of disguise and bluff, mixed in with nigh-immunity to divination and mind-affecting spells, as well as some sneak attack. Does anyone have any advice?

*Edit for spelling