After completing the Magic School section of the game, you will go on a search for Zellini and it will lead you back to the underground library. On your way there, you'll encounter this enemy.
If you picked up Edessot's Supernova move (by finding the Pyro Pages book I mentioned in the level 5 gather achievement), just send Edessot all your gather and have him use Supernova. After 2-3 Supernovas, Boris will be dead.