The key to expert is to avoid friendly fire, and corner EVERY swarm. Friendly fire can wreck you so fast, and with zombies doing 10-20 per hit, swarms MUST be managed with corners or you're hosed.
Also, very helpful to have at least one guy watching behind the group.
Normally in vs. I go shotgun for T1, but I find that going submachinegun/assault rifle is best on expert because it's easier to avoid friendly fire, and if you do hit your team it hurts less than a shotgun blast.