action shadow_act(){
     VECTOR temp;
     var distance_to_ground = 25;//set this yourself
     set(my,PASSABLE);
     my.tilt = -90;//or 90
     while(you){wait(1);
               vec_set(my.x, vector(you.x,you.y,you.z-distance_to_ground));
     }
     ent_remove(me);
}

action enemy_act(){
     //begining of action
     ...
     ent_create("sShadow_Enemy.png",my.x,shadow_act);
     ...
     //before the while loop
}