Have Broom, Will Tavel

When Susan Crone Takes up the post of village witch, she gets a bit more than she bargained for. She Ends up with a klutzy apprentice, and goes up against Damian, the man-witch. Soon Susan finds herself on a journey to stop Damian's power-mad father from destroying all the magic in the world.
Along the way she confronts an army of mermen, a city stomping monster, a shape-shifting demon, flying monkeys, natives looking for a sacrifice for their volcano god, and some munchkins dealing "Magic Herb". Of course she also has to deal with Damian, and his rather loose definition of "celibacy".