The events sheet looks something like this:
System: every 6 seconds enemy: spawn bullet
enemy refers to all the enemies in the map.
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What you could do is every: random(4,6) seconds enemy spawn bullet. That would make a bullet spawn between 4 and 6 seconds. Or when the enemy is spawned or when the game starts you could give it an instance value of random(0,6) and have a timer subtracting that value every second. When it reaches zero you could spawn the bullet and reset the timer to 6. Hope that helps.
You have to "pick" the instance you want to act on or else all instances will be picked.
In short you have to figure out a way to single out the instance you want to fire separate from the other instances.
In my prototype space shooter in my sig I used the technique mentioned by currypuff. My basic enemy ship each shoot at a random time.. it works for the most part. Some of my enemies, however, are picked by distance from player and others using a collision with an invisible sprite barrier... you have to figure out what works for your specific need.