In the Ocean palace a couple of Mystic like creatures exist. Sorceror recolors? So while Magus is a possibility, I'd say probably Lavos. Since in the Ocean Palace Lavos is strongly present and everyone(I mean EVERYONE including Dalton's goons)seems to be fully capable of using magic.
Then as Bekkler stated there's the Mystics in Dorino Mountains. Taking them without magic and the Medina mystics having magic into account,(not to mention the non magical mystics of the cathedral)I'd say since apparantly Magus is summoning Lavos and so I'd imagine his castle being very filfthy with the very nature of Lavos' presence, that's probably why they're the only mystics to have magic.
But that then brings up a thought of how then can Magus use magic even when he's away from Lavos? I'm guessing since he was born in Lavos' presence in Zeal, and grew up with it, it attatched itself to him and almost became like a part of him(hence why these characters don't have to use elements but Cross characters do). Magic flows within his veins.
So then what of the Zealians who lost their magic when they lost access to Lavos? Magus was Janus and Janus was upper class royalty. And as we saw, it seemed royalty got to come closer to Lavos than anyone else. This would explain the power behind Schala, Queen Zeal, and Janus/Magus lasting without Lavos. This could also help explain how the closer they were to Lavos, the stronger they became(Magus is tougher as a boss than a character, probably cause he's summoning the thing). Queen Zeal was strong already but not as strong as when she flew the Black Omen which was practically powered by Lavos, thus allowing her to become so strong she became that face and hands thing. And Schala we all know how she was extremely powerful, and then she merged with Lavos and boom. All powerful destroyer of space and time. Afterall, as Lavos' final form shows, Lavos was just harvesting us and using us to evolve itself. When it analyzes our species it copies us and becomes stronger, so what if it had an actual life form in person to study, analyze...and possess? This would help explain that.
So for that, I'm gonna say Lavos is one reason the mystics can use magic.