>Enemy On collision with Rock
AND Enemy is OnScreen --- DO THIS
As far as I know, the engine works in a way that it checks collisions for every object every tick.
The "Enemy On collision with Rock AND Enemy is OnScreen" event will only trigger when an enemy is collided with a rock and it happened on screen. The engine still checks for collisions for every object (not sure for the objects off-screen), but you can turn off this by setting the "Collisions" setting to "Disabled" in the object's properties.