Play on easy, grab adrenaline and whatever weapons you prefer. As soon as the level starts, hit the radio, then the button to lower the bridge. Once the bridge has lowered, use the adrenaline and just run in a straight line, only stopping when you absolutely need to (i.e. when zombies are right behind you and you're getting slowed down by their attacks). So long as you keep an eye out of for tanks and don't stop running, you should make it all the way within a minute or two. If playing solo, the AI will try to keep up, so you don't have to worry too much about them slowing down your final time.