The way I beat it was opposite of Alex. Sorry Alex.
Once the guy starts to download information from the computer, Stay on the stage, and check/send your teammate to the left of the stage on the other side of the computer. He should be posted up behind the crate, so he is protected. The difference is that instead of your teammate using the machine-gun... YOU USE the machine-gun. Like Alex said, he sucks, and barely kills anyone anyways. I found that he does the most help at the left side, while you man the machine-gun yourself. This allows for more accurate(if you're good), heavy firepower that can make short work of the enemies that pop out in the seats, the balconies, and doorways.
Make sure you keep your eyes open, and kill everyone that shows up. You don't want them shooting at the dude downloading from the computer, and you definately do not want bullets blowing the computer up(usually happens first- which fails the mission). You doing the machine-gun yourself allows you the best control of the action/firepower, ability to think on the fly, while your teammate backs you up.
NOTE: Once you beat this part, make sure you watch out when you climbed the ladder up. There is a sniper that will kill anything that moves up there. Shoot at the doorway he's at, from the side you went up.