In my game, I've had a boss encounter I've been leading up to for 16 levels now, where the players will be fighting a gnoll leader named Kolaar who has managed to unite the gnoll and bugbear tribes, teach them agriculture, strategy, and trade, and has led them to conquer key cities. Kolaar's secret was that he had an invisible spirit named Oal that could spy and do tasks for him in addition to casting spells to defend him or to serve him.
After a defeat that ended in Kolaar's forces losing control of a port city, he was able to salvage his forces and take them into the Underdark. By inspiring revolt in the drow slaves coupled with strategic attacks, Kolaar's forces have seized a drow city. In the last adventure, the players met a human wizard outside the city who claimed to have studied here and wishes for this threat to be gone. With his help, they gain the alliance of some previously-captive dwarven spirits who go and kill Oal.
Now defenseless except for his tactical mind and fighting prowess, the players can move directly against Kolaar. I am sure you can see that I want this confrontation to be epic and interesting.
Here's the problem. One of the players is a psionicist. A psionicist who likes to use the psionic equivalent of Disintegrate. Were he a spellcaster, I would have Kolaar use tactics like a stone with Silence on it (much like drow have Darkness cast on stones wrapped in clay) to help disrupt or hinder the player's spellcasting. But as far as I can tell there is nothing short of stunning or knocking the player out that can disrupt psionics. And even then, this feels wrong, because I don't want the player to not be able to do anything. But if I let him do something, he's going to sever the boss' spinal cord or teleport its brain to Nebraska.
Any ideas? I don't want to just say things like, "Oh, well, your ability doesn't work for some reason." That seems cheap.