We spent about 20 extra minutes last night backtracking around Wailing Caverns as well (Ab, was it my group you were in?
), trying to get to Serpentis. I kept seeing him on my minimap (Hunter track humanoid skill), but I could figure out how to get to his elevation.
Someone mentioned a site that had a map, but I'm guessing it's the same one LE found, and while it does indicate where the bosses are, there's no real means to tell how to get to them.
We did end up figuring out what we missed (one turn we hadn't taken), and eventually cleared it, but it was almost to the point where I was ready to give up on the quest. Fortunately our group stuck it out and we did eventually finish it up.