The Dragon and the Apprentice: A Wizard’s Wager (The Chronicles of Sir John the Worm, Wizardslayer Book 1‪)‬

A dragon and a wizard duel every ten years to see who is stronger. Each time the duel ends in a draw. Realizing they're growing too old to continue fighting with one another they make a bet. Each must pick and train an apprentice. In ten years time, the apprentices will duel instead.