hawkeyefan
Legend
It would be easy enough to drop her CR down. The big ones are the boon of recovery and the shield spell. The Witch Queen as shield as an at-will spell so she essentially always has a +5 to AC. If you keep her AC at 24 (swap out the shield spell for another spell) and remove the boon of recovery (which I factored into her effective HP) then I think she is CR 20-21. If you then drop meteor swarm, she is down around CR 17-18 I think.
Thanks...she's a bit off as far as an actual opponent for them, at this point, so I think which one I use will depend on when and how they face her, and how the campaign goes. I'll keep those adjustments in mind if I think they are needed.