I really think it is fighting the frog like creature inside the cave. I mean look at that dumb ability that he does, hopping up and down shaking the ground. If he does that inside the cave wouldn't you think that maybe he jars something lose, cave in a wall, exposing a chest? I think he has something to do with it.. Heck it might expose another miniboss, who knows. dying to try it.
It's a long way to pull him, but I think is doable. I guess you have to go and clear the other trash first then come back and pull the Gargantuan Lobel to the cave. Not sure my guild is going to be up for this