I've been searching some more, and it turns out, the "mancy" prefix is actually derived from the greek word for "divination". I know it is typical to name them like that, and since I never knew what exactly it meant, I was fine with it. But now... I'm not so sure, there might be better words.
But that helps me where I was stuck on getting the -mancers translated to German. I am not going to go with a mancer or divination translation, but simply "Flammenwirker" for a Pyromancer, which I would translate back as "Firecaster".