13. glitterdust / faerie fire (I see that was already mentioned...)
14. Clarity of Vision (skill trick)
15. Do him one better and go ethereal!
16. Surround the NPC(s) inside a wall spell for protection and to block line of effect. If he tries to get in or destroy the wall, you'll likely know where from or which direction.
17. Not sure, but running around w/ a Greater globe of invulnerability till you get lucky?
18. layer the ground w/ caltrops, broken glass, or sand so you can hear/see where he is (worthless on flying PC)
19. Win initiative. Either a) cause a slow effect on him, so he can't both move and cast; or b) place a "targeting aid" on him. Examples - Spiritual Weapon, continuous bleeding damage (like the Invisible Blade class feature, among other sources).
20. Use that new 5th level cone spell from Complete Mage that bars a school or subschool from working in its effect area.
21. run away and come back (it's only 1 round/level duration!)
Edit: The spell is actually an emanation from the caster and 4th level. It's called Otiluke's Suppressing Field