Radiance I can only be learned from the exorcist after you save her from Mountainreach, which requires you to defeat the day of sloth boss, making it a new game+ strategy.
What day are you at? Are you already using the waterskin I spell and/or antimagic coating? Are you hitting them with light spells every other turn to gain tactical advantage? How high are your luck and agility? Are you targeting Belphegor or Luciela first?