Witches of Kregen by Alan Burt Akers

When his new army was ready to march against the witch hordes, it rained frogs! It was a veritable heavy bombardment from empty skies! That's the sort of thing Dray Prescot is up against in the war of the Nine Unspeakable Curses! Dray was struggling to gather together his shattered empire when the witchcraft hit. But he has wizards on his side, too, and very soon it becomes a battle of sheer courage, quick wits, and fast flying. This was more to his liking, for he knew that this time the Star Lords might be on his side. Not that he could rely on them, for they were just as likely to toss him back to Earth for a crash course in the old world's learning!

Witches of Kregen is the thirty-fourth book in the epic fifty-two book saga of Dray Prescot of Earth and of Kregen by Kenneth Bulmer, writing as Alan Burt Akers. The series continues with Storm over Vallia.