Outside the entrance to the building that houses rockets mcdickface in between him and the tank if you go to the right of the building you can actually just hop inside of the walls. Before I tell you how pls note that you cannot use this at the valaus part as there is an invisible wall. Underneath the right side of the building are 3 giant caltrops all in a small clump. Go and stand right around them and look up. (It's not directly above you) but there is a large rectangular hole that you can jump up into. Niw just explore around and I have tested that armour peircing rounds do work in this area and can be used to shoot the cabal outside of it. Also if you get up on top of the hallway you go into the building with (while remaining in the walls) you can jump up high (may need increased height/bones of eao but I'm not sure) and throw a grenade out down at the cabal as you will be outside the walls. However as you start to fall the invisible walls will pull you back in, so the only way out is where you came in :)