Can be done a few ways but here's how I'd go about it.
Set the enemy spawning events under a group.
On enemy destroyed, add one to a variable.
When that variable equals ten, Set group 'enemy spawning' deactivated, create object power up.
On collision with power up, wait a couple seconds, set group 'enemy spawning' activated. This removes the need for a variable to determine whether or not enemies are spawning.
Make sure you apply the "trigger once when active" condition so that only one power up spawns.
Also, I would do a separate event like Powerup - on created -> set bullet speed to random(10,80).
Is this what you're having trouble with or something I haven't picked up on?