One tip:
Do apply the "Always Simulate" behavior on enemies.
The huge blue things, that are hard to kill, when they touch the left side of the screen they do simple disappear. It seems strange.
To avoid that I do recommend using the "always simulate" and then you control the "kill" of the actor.
For instance...
On a On Drawing Event of the enemy actor you do:
If enemy.X value < -50 (out of sight)
enemy.kill.
That way it will pass all the screen, will not disappear and then you will kill it, avoiding performance issues.
I know the above code is not stencyl, but the logic is the same.