Thanks for the suggestions, tyckspoon. Spiritual Weapon is one I hadn't thought of, but for a Cleric of Kossuth could be pretty good: Kossuth favors a spiked chain. Getting 4 attacks a round with proficiency in an exotic weapon isn't something you normally think of for a Cleric.

My main need is to avoid getting the Cleric killed, as it's a significant opponent NPC for the campaign. Beyond that I want to diminish the power of the PCs as much as possible, so they won't have enough resources (people, magic, supplies, whatever) to win the encounter when they reach their final destination.