Zale said that at the end of it, we should be able to comparatively cast most spells.
The way I'm handling it is Billy's god literally taking him away to sort out his issues, and give him some pointers, and sending him out on a holy mission. Since Billy worships a god of death, he has literally all the time in the world for training.
I'm dusting off the good old hyperbolic time chamber cliche. God help me, it's the only way I can think of that will make sense to me. A literal Deus Ex Machina.
Marv, incidentally, I apologize. It seems I showed up to your all wizard game having rolled a cleric.
Hey son, wanna' learn how ta' make witch balls?