What about Shield of Darkness?
That is a 50% miss chance unless the PCs have a way to see through magical darkness.
Find some large animals with grab and burrow, turn them into (fast) zombies, and have them burst through the floor and grapple the party. Prep floor with appropriate illusions first.
Stand behind a wall of human skeletons that are wielding tower shields, they will provide total cover, make a lot of them. It will take time for the party to whittle them down if they do not resort to AOE, sprinkle in prisoners to make them think twice about resorting to blasting.
Fatigue/exhaustion effects can seriously nerf the party, and you are already controlling the weather, so you can make them deal with massive heat/cold issues, or force them to spend resources on endure elements.
Have some minions with high ranks in intimidate try and demoralize them, it may not be much, but every little bit helps.
Stone Shield may also help.
The debilitating strike from the rogue mooks will help a lot, give them the scout archetype to guarantee them being able to get at least one sneak attack attempt a round. For more evilness also give them the rake archetype and the intimidate skill unlock and just make the gunslinger spend a couple of rounds running away.
Or just fill a clay pot full of catnip and toss it at the catfolk gunslinger....
