Zombie server map

Hey I'm also thinking of adding a mini-game like that to my server but does anyone know of a plugin that makes only zombies spawn? It needs to work with Multiverse so it only enables it on certain worlds.

Well, you should add circuits that spawns the mobs in rounds/waves so that the player can regenerate health when taking small breaks between each round. You can use command blocks to make doors that will unlock after achieving a certain amount of kills or rounds completed. That way the player can progress more instead of being locked up in the same spot all of the time (that will become boring after a while).

Hope that helped!