Use your grapple tool to pull the Librarian in close for a few hits while it is stunned. Avoid all dark magic by dodge/rolling the projectiles and not standing in the flames. Clear out the minions after they are summoned. After the Librarian vanishes, be ready to move to the middle of the platform just after it slams down. Finish off the Librarian with the Fire Wand when it is low on health. Lastly, collect the Green Key to unlock the trophy and return it to the temple.