I'm stuck. If I try to mow them down shooting-gallery style I run out of ammo before the level ends. If I try to let them bunch up so I can save ammo and use the bombs they manage to break down the gun. Either wya I end up on foot, and get overwhelmed. I tried mining the entrances with poison grenades, but for some reason the gas dissipates as soon as I mount the sentry gun - bug I assume.
How did you guys get through it?