SpinFX and I also had the idea of zombie hoards I recon if they behaved as a unit it should be because they were a unit when alive for example a military squad, football teams or firemen, maybe a team of doctors with the nurses too. Another idea to make them more deadly (besides numbers) would be that the AI can all see what the other AI in the group can see but will still only attack one target at a time before moving onto the next.(depending on server loading) also maybe make them able to track via smell (again depending on server loading) and will do a "jog" when tracking a scent so that they will catch a walking player, but not a running player and it should then be possible to loose a zombie hoard by running through water or using an item to cover your scent (ammonia? cleaning products? smoke nade? formaldehyde?)