I'd do something like this.
Make an array, each entry corresponding to a monster type.
Then try something like:
global variable monsterIndex = 0
every 2 seconds:
if array.at(monsterIndex) = "zombie"
--> create zombie
if array.at(monsterIndex) = "ghost"
--> create ghost
if array.at(monsterIndex) = "squid"
--> create squid
(etc, for all the monsters)
Add 1 to monsterIndex
Do you see what I'm getting at?