Retreat is not defeat. Know where you are going to retreat before you enter a sn´ell. Retreating from where you came is often the best option. Knowing the area around you is good, and generally any area will have at least some safe spots. Or some that are safer than others. All in the group is better off retreating together, heal up and face the beasties once again as one, instead of fleeing in different directions and fall all scattered about. Usually it´s best to retreat to the sne´ll you all came from, since this sne´ll will be clear most of the time, but that is not always the case.
If the one who leads yells Run east!, it means full speed. For all. Get the hell out of there as fast as you can. The strongest fighter or healer will always cover the rear, and the longer you delay the longer she/he will be hit. Get out of there!
At the next sn´ell, don´t just hang around at the edge. Run to the middle, since there is less chance of the beasties to come after you then. You also buy some more valuable minutes to get healed, since the beasties have to cover some distance to get to you. Sometimes, even two seconds of healing can be enough to survive.
I remember a battle in DC. We were fighting some wailing spirits a bit deep in the tunnel, and I was at front, bricking with healers backing me, the other fighters moving in and out. We didn´t want to give up the fight, as Wailing Spirits near the entrance is bad news. They don´t move much, so even if we did retreat and healed up inside the tree, we would have a hard time getting inside again. I was constantly on yellow, and some hits would get me on low yellow. I didn´t brick them so good then. Then we backed up a little, took some steps while the healers were healing me, and in that little timeperiod - the Wailing Spirits had to move also. That little time frame there was enough. We did this dance for awhile, until we finally had conquered them.