I use the SPRITE - PICK ONE AT RANDOM event. Then set that to spawn a bullet, which was set to fire down the screen (in the bullet options). The problem was, it fired all the time. So I also added a RANDOM condition to the event so that it didn't. I found that getting it to choose a number between 1 and 100 and allowing it to fire if it picked 50. You can add a condition to an event just by clicking on it when you've created it, a popout appears allowing you to set it.
This is how it looked in Construct Event panel.
Great peace of software BTW. I'm lucky enough to be able to search through the options and commands but I imagine there's loads I'm missing. Hoping to help with the Wiki if I become comfortable enough to pass on my knowledge.