Thank you very much both of you gentel men!
I tried to implement BluePhaze solution, indeed I asseigned the bullet behaviour to my enemy(which is supposed to move above a wall), then I added a condition just like that:
(to perform the left movement)
System -> Every tick -> myenemy : Move 3 pixels at angle 180
until now, it works!!!
Then, I want to make it back when he arrived to the wall limit( the right movement), I want to compare the wall X and my enemy X coordinates:
myenemy -> X <= wall.X : myenemy: Move 3 pixels at angle 0
but the problem is that myenemy object didn't turn back :(
What was my mistake?