Using bullet behaviour should work fine?
If you set the angle towards the player, and set the speed to something like say, 400... it will fall if it's above the player.
If the problem you're having is that the falling thing goes back UP again after it goes below the player, you need to set conditions on the movement. If you only want the thing to move and aim at the player if it is above the player and not below, then make the condition something like;
[Compare Y position of falling.object -> is less than player.object]
--- Set angle to player.object
--- Set speed to 400
I guess you should draw a picture of how you want this to work in your game though, because I still don't really understand what you're trying to do.
The problem whit that is that if the sprite is above or on the same Y as the other sprite it won't aim anymore. Making the sprite only aim at the other sprites X would work. But i don't know how to write that expression. I know the expression for making the sprite aim at both the other sprites X and Y but not just X.
angle(Sprite.X,Sprite.Y,OtherSprite.X,OtherSprite.Y) is the rigth expression for aiming at both the sprites X and Y. You can use it in different ways to change the ways the sprites aims. For just making it aim at the other sprites X though, even if removing the Y parts of the expression seems rigth it doesn't work. So how do you write it rigth?