The characters in my campaign are 11th level, and they've only recently been faced by bosses of 14th level or so. And recently they've faced evil clerics, evil magicians, and evil mentalists.
What I found was that it was the lowest level spells that were most effective. Dark Stunning is brutal, and puts a character in a position to whammy with a more powerful spell in the time they're stunned. Suspicion and Trait Subversion can have long term impact on the party, but even in the moment it's cast, Suspicion can make a mess of a battle plan, as the target doubts the intent of allies and begins to deviate.
When the party is resisting well, high level spells can be a risky use of power points. One well chosen low-level spell, cast with no prep, can be attempted several times for the same PP cost.
