From my limited experience, when a couple humans manage to reach the alien base intact, getting to the OM hasn't been too much of a problem. (Especially with a few Lucis in the mix)
For the most part I've seen the human team get somewhat stuck in their base, unable to reach the alien base at all with any sort of force. However, my experience is all with AKKA's high player count..
Actually, that is because their exit out of the base (the door) is too small, so only 1-2 huans can actually go out at a time, plus they block the fire of their teammates which (if FF on) damages them, and/or gives aliens a bulletshield and some evo points, so my sollution is to put a big door that opens slowley and closes slowley (like in Karith) os humans will have the time to stop shooting and that more humans can go out at a time.
Also, i've seen a small grey vent going through the wall and around the human base, this could be a nifty path (only) for alien dretches to get closer to the human base rather than: 1. block the bigger aliens and somewhat feed the human team whilst they try to take the offensive. or: 2. they camp out of sight in the corridor hoping that they will get any passing human, whilst the human team mostly has broken through the front whith luci's, Bsuits and pulse rifles and advancing through the level, and dretches aren't suited to stop that.
So the vent should start at the Resevoir, go through the wholse human base spot, have 1 hole in the vent to go through somewhere whilst the vent is going , a vent-exit in the human base, and ofcourse lot's of turns.
Humans shouldent be able to go in, ONLY dretches (don't worry, dretches can't evolve if any human (structure) is close to a vent), but the vent-exit (not the hole in the vent) should be defendable with a turret or something

.
Anyhow, that's just my 2 cents :wink: , comments / reply's are appriciated

.