Just keep on running and follow the waypoint on your HUD untill you reach an appartment building. I've noticed throughout the game that if you don't keep advancing, more enemies will just show up and make you run out of bullets. Once you advance to a certain point your buddies will move on and the enemies will stop coming.
I finished the game just 30 minutes ago, releasing the Arcade game mode and the abillity to enter cheats in the pause menu. If I'm not too busy tomorrow, I'll try the online multiplayer.