I think the only thing one can gather from the games is that Schala must have a wide variety of magic skills, just like Magus. I suppose that one could make an assumption that she may use shadow magic, as it seems like that type of magic is popular in Zeal, plus I don't believe Lavos would react to much to other forms of magic other than shadow (it just seems to fit the cliche of wizards summoning huge evil dudes in games and anime. Magus summoned Lavos with shadow magic (one could assume), Schala was activating the Mammon Machine with some kind of magic, I just assume it is some type of shadow seeing as how the only other frame of reference to someone "summoning" is when Magus attempts to do so). But I think the easiest answer is that she is just skilled in many elements of magic.