Try As I Smite by Abigail Owen

Alasdair Blakesley is the head of the Covens Syndicate. He shouldn’t need to look beyond the witches and warlocks he governs, or his own abilities, to solve any problem. But a demon infestation means he can’t trust anyone who may be possessed. The last person he wants to ask for help is also the only person who can fix this, so he sucks up his pride and storms into her office.
And she turns him down cold.

As the owner of Brimstone Inc., Delilah’s passion is helping others with their supernatural problems. But Alasdair is the last man Delilah wants to tangle with. The infuriating man sees too much and demands even more. And did she mention the way he sets her body on fire?

Not that it matters—demons are the only things with which she cannot interfere. Too bad a higher power steps in and sends them both on a crash course of each others’ pasts, presents, and futures. How is a Christmas Carol nightmare supposed to solve his demon problem without breaking the rules she’s bound by?