I have beaten him on the left other weeks, but this week right of where you come out worked best the two times we downed him. The tunnel on the right gives you protection from boss splash damage and it allows you a chance to get the jump on a bunch of the adds as you can kill them right as they come out the door. Only two entrances to cover and less hobgoblin exposure. Get jawbreaker buff and group up in tunnel for the containment bubbles, pop out and shot rockets or whatever ya got. One time this week the boss teleported down on the left and was almost on top of us, that was fun. So anyway, both places work, just giving you another option in case you are getting whooped.