Normally I did not have this problem, so maybe it's some issue with your configuration, but OK, I can offer you a solution for your situation.
Create a GLOBAL variable SHOOT = 0
SYSTEM Every X seconds (e.g. every second)
AND when SHOOT = 1
ACTION = Enemy shoots
So regulate their shooting by turning ON and OFF the SHOOT variable.
A cool tip is to say every X seconds (e.g. 0.5)
SYSTEM pick a random enemy instance
This creates a more realistic game rather than all shooting at same time.