This is story related but can be reached through the Gor'Castle Parks entrance. Take the boat ride, and skip the cut scene by pressing Y. Go to the right until you go down stairs into a hellish area. First part of boss is on ground second is in the air.
If you are playing in someones game who already beat Beelzebaal then he will not spawn and you will either need to go back in your own game to beat him or join a game that has not beaten Beelzebaal.