I know where you are ray... you have to use the Tripods and sit it out defending yourself from a few waves of soldiers, if you want info on the best way to get through that part you can visit this thread
I got a hint, and it worked. Set the guns up in their lockers. You have to take em out set em down and get around to the front so they're pointed the right way. Make sure there is one in each outer locker and the third on either side. Get into the remaining locker and crouch, and wait. If you get bored stay crouched and go exchange some fire and then get back into the locker. When they come in, it's a turkey-shoot. Most walk right past, if you're quick on the trigger even the ones who try and engage will lose. I've experimented with this and even scuttled out far enough to roll grenades. But a couple of times there was never a fight. The comment about setting up the last gun being the trigger is spot on.