Well, I would recommend using those monsters to level up a bit, but when you're done, go up the small hallway to the right and wait until all the monster start coming to that side. Then rush back around and go up the left side fighting only the ones you need to. I would also recommend waiting until you finish up in the Thief's Guild so your persuasion points are high enough to get the Zeon to give you the treasure. As far as I can tell, once you complete the quest, you can't go back.
