Why don't any of the characters have ranged weapons, magic items that let them fly or otherwise cross the gap, and failing that why don't any of the casters in the group prepare spells like conjure animals, fly, dimension door, teleport, or the like? These are 11th level characters, not 3rd level, there are a lot of ways to cross a gap to engage an enemy in a fight. Exactly what to do will depend on the party makeup and magic item availability, but there are a lot of ways to get across the gap to engage the enemy in melee. And failing that, 11th level characters should at least be able to afford bows so that they can do something other than sit around after tossing javelins.
This was a bunch of young, new players. I am the experienced one in the group. So I really think it is my own fault (and the adventure designers

And now that I think of it the rogue did have an item that let her summon animals. She summoned a few giant birds and flew the barbarian across. Which was when I decided it was a good excuse for the BBEG to teleport to the other side and summon a whole bunch of minions. The birds flew the barbarian back so he could get back in the fight.