Create a stack of 1x1's in an inaccessible place, equal to the amount of zombie spawns + 2.
Name the top one "zombieRespawnFire".
Name all the others, apart from the bottom "zombieCheck".
For every zombie spawn:
Set name to "zombieSpawn"
onZombieDeath -> Self -> setVehicle -> NONE
onZombieDeath -> <NAMED BRICK: zombieRespawnFire> -> spawnProjectile -> [0 0 -90] [Radio Wave] [0 0 0] [1.000]
For every zombieCheck brick:
onProjectileHit -> Self -> disappear [-1]
For the bottom brick of the stack:
onProjectileHit -> Minigame -> centerprintAll -> "<color:FF00FF>Players win! Resetting in five seconds."
(Delay = 5000) onProjectileHit -> Minigame -> reset
(Delay = 5000) onProjectileHit -> Self -> fireRelay
onRelay -> <NAMED BRICK: zombieSpawn> -> setVehicle -> Zombie Normal
onRelay -> <NAMED BRICK: zombieCheck> -> respawn
When a zombie dies, it will not respawn. When all are dead, the minigame will reset. You might need to edit it slightly if you want more than one kind of zombie.