The challenge with Dominic isn't defeating him, it's finding out firstly that anyone is mentally controlling half of Dementlieu high society in the first place, and then secondly finding out who it is.
It depends a lot, I think, on where you want to be on the realism-versus-horror scale.
Realistically, the PCs should
probably never be able to find out that Dominic has mind control powers. He's too clever to reveal himself and likely won't have a need to hypnotize the PCs directly. The only time there might be a hint is if the PCs plan on doing something that Dominic doesn't want them to do and all of a sudden friendly NPCs refuse to help them and the police come to arrest them or drive them out of town, even if those people had been on the PCs side up until that point.
Horror-wise, however, it could be quite easy to at least reveal that someone is mind-controlling everyone, if you don't mind going cinematic.
1. Dominic wants to get the PCs a message. It's delivered over the course of a few hours by numerous different random people the PCs meet, each giving a different part of the message. And each person knows something about the PCs that they couldn't possibly have known. Because in 5e, at least,
dominate person lets you look through your host's eyes, and I see no reason to not give Dominic that ability as well. Some NPC saw a PC doing something, and now a completely random one knows as well. Whether they remember when they're
not being mind-controlled is another matter. But Dominic remembers.
2. The PCs are somewhere, like a tavern. Suddenly, every NPC blanks out, gets up, and walks away. If the PCs follow, these NPCs are doing something truly odd. If one is awakened, they look around curiously, then shrug and go back to whatever they're doing. And since it's Dominic, the something odd could be anything from attacking one of The Brain's bases of operations to attending an artistic event because Dom's crushing on the artist and he wants her to have a perfect opening night.
And of course there's the matter of Dominic's protectors, the ones who actually guard him from physical threats. [...] Also, who ARE they? PCs are more likely to get in combat with them than with Dominic himself, if the book is going to give us any stat blocks, theirs are surely more useful than his.
As they're totally dominated and Obedient to him, Dominic's minions would also be "willing" to throw themselves in the worst peril for him. So the PCs are going to confront Dominic and all of a sudden dozens or scores of townspeople throw themselves on the PCs, attempting to murder them. Some players may be fine with killing townsfolk, but others (including my table) would balk at that. And since they're technically innocents, killing a controlled townfolk could be worth Dark Power checks.
Plus, his minions could do little things to harm the PCs. Jean the innkeeper always seemed to like you and he swears up and down he doesn't remember poisoning your stew, even under a
zone of truth spell.