In this area there is a very narrow strip of land that you must pass over to advance. It's before you get to the rock/rope climbing area. My suggestion is the the spawn point placed there be moved a little bit one way or the other. Currently the spawns there can entirely block any passage. If you are stealthed/invis you just cannot get past monster models no matter what you do.
While I like the idea of them holding up the path, completely blocking it is a bit much. The earth elementals especially are culprits for blocking up the road. The smaller stuff you can usually squeeze past. Just a small move in the spawn location would maintain the spawns effectiveness while still providing at least some small means of passage/escape.