There's a point early in the level where the path branches left into a crawlspace, or right into an open area. Take the right path, and at the end, you'll get into a room with Sentries fighting Cross-Species (I know this doesn't help you any, sorry).
On the wall opposite from you, there's a red/brown wall with white markings (almost like a cave painting). Take a picture of that.
Actually it helps me alot since it sounds like your talking about the area where the fighting is going on and you can hear it clearly well before being able to see the combatants. I already beat the level but I at least know what to look for now when I replay it and where to look.