You can use System > Pick Random Instance
In case you don't want to spawn same place, you will need variables:
- spawnCounter: it is instance variable of spawn object.
- spawnLevel: Global Variable in Event Sheet.
Each time you spawn an object, you can add 1 to that spawnCounter.
When you pick random instance, you need subCondition to check all instances which have spawnCounter equal the spawnLevel.
When no spawn object has spawnCounter equal spawnLevel, you add 1 to spawnLevel and keep going.