Just to be fair, you need a -16 to lock a solo with 100% certainty.
With a -15, on a 20 roll, you get: +5(solo)-15(orb wizard)+20 = 10. A 10 is a succesful save.
But even then, using Earthroot Staff or Cunning Weapon/WotST, spell focus and Phrenic Crown alone, you easily hand out saves that most...