You can miss this boss if you skip the tutorial, so be sure to play through it.
At the end of the tutorial you come across Behold and must defeat it to continue. You can beat him legitimately with your new found magickal prowess or be a bit cheap at hit the rope of the left wall to instantly crush the boss under a chandelier. If you choose to fight him with spells then you should use lightning based attack as Behold is weak to them.