My 2 servers:

[Zm_B] 4COOP Epic | GunMenu 94.23.222.118:27500
server in normal difficulty but crazy (need a good team) All specials infected respawn every seconds,more waves and bigger,more hp for tank and a gunmenu(all weapons for free when u want,just press twice on reload to open it) U respawn a tank if u kill the witch

[Zm_B] Epic Loose 83.142.49.163:27015
This server is just for expert mod but he is harder(need a good team too) More specials infected,more waves and bigger,3 tanks,and a buy system.You will respawn tanks if u kill the witch

U can create a lobby who go on it directly,use this command:
mm_dedicated_force_servers 83.142.49.163:27015
But the server must be empty

Join my group if u want find them in Steam Group Servers List:
http://steamcommunity.com/groups/ZombiesBastards