Pretty sure this will be moved to the Pathfinder-specific forum.
There's really nothing for cultists in that book. There's only warpriests and shamans when it comes to divine spellcasters, and neither are more powerful than clerics or oracles when facing a gunslinger.
Gunslingers are obviously weak in melee, so find an aberration that can teleport or dimension door as a swift action. I don't know if there are any, or more to the point any around CR 8. As DM, you can custom-create such a monster. Now the monster can teleport into the gunslinger's face, and any time the gunslinger retreats, the monsters gets an AoO. If the monster is faster than the gunslinger, then the gunslinger is fairly screwed.
The gunslinger is not well-balanced. You need to build every single encounter to nerf the gunslinger if you want to challenge them, which is a pretty good indication that the class has problems. There are numerous spells that can boost touch AC, but not by enough. Try spells that give miss chances instead, such as Displacement. (Mirror Image effectively does this too.) I suppose an "aberrant alienist" sorcerer that spams Displacement on allies and summons lots of creatures (including, of course, some right in the gunslinger's face) would work wonders in this situation.
