Things to know.
Gamemode_Zombie-
1.) Don't set all of the zombie spawns with selected zombies.
2.) Do set the spawns down in random places either scattered and/or grouped together.
3.) Do type /hidezombiespawns after setting down spawns
4.) Do enable friendly fire, and the director for the minigame.
5.) If you want something to happen at a certain point, event a zombie attack, like so-
[OnActivate/OnPlayerTouch/OnRelay/etc.(whatever you want it to be)] >> [Director/Minigame (?)] >> [Start]