What challenge rating (‘CR’) is a Medusa given the textbook NPC-Diviner-Wizard casting ability?

I wanted to give a recurring villain / anti-hero medusa some (15th level) wizard abilities, a.k.a. the N.P.C. Diviner as featured in Vole’s or Xanathar’s or Tasha’s or somewhere. She can use this NPC class to ‘reset’ her Portent ability with every (re)cast of any 1st lvl+ Divination spell. This is handy for Petrification management for both attacks &/or mistakes.

Basic question: what is her total Challenge Rating? Clearly not 6 (‘Medusa’s CR’) + 8 (Divination Wizard’s CR) = 14 (sum total). It seems the Monster Manual, the DM’s Guide and Players Handbook (and other guides) all offer tip-suggestions – yet I remain challenged by Challenge Rating.

Why I Require This ‘True’ Challenge Rating:

With a ‘true’ medusa-wizard CR the P.C. group will:

  • gain the ‘appropriate’ treasure(s) considered fair &/or balanced;
  • also gain matching-appropriate ‘x.p.’ rewards, and;
  • (most importantly) the player-group will be slightly less bitter encountering a ‘monster’ with approximately an arch-mage’s firing power backed by with her Clone-able immortal body.