My advice would be to soften them up with other encounters first. A fresh party will tear apart even a high refresh villain, but if you send that villain up against a party that's already dealing with several consequences and has used much of its resources, it's a much better challenge. Hell, it's what Butcher does--how many fights does Harry get into that can be summed up as, "He would win easily, if he wasn't tired and beat to crap"?
In my game, that's basically what I've done for the last big fight of this scenario--threw ghouls, vampires, and cultists at the party first, so now they're facing one reflavored Toad Demon and an injured sorcerer, but the wizards have already soaked up half their mental stress, one of them's got two consequences, so does the tank, and their enchanted items are nearly depleted.
Alternatively, instead of inflating refresh, inflate the stats.